Add support for additional text encodings.

This includes, but is not limited to, UTF-16, latin-1, GBK, EUC-JP and
Shift_JIS. (Courtesy of the `encoding_rs` crate.)

Specifically, this feature enables ripgrep to search files that are
encoded in an encoding other than UTF-8. The list of available encodings
is tied directly to what the `encoding_rs` crate supports, which is in
turn tied to the Encoding Standard. The full list of available encodings
can be found here: https://encoding.spec.whatwg.org/#concept-encoding-get

This pull request also introduces the notion that text encodings can be
automatically detected on a best effort basis. Currently, the only
support for this is checking for a UTF-16 bom. In all other cases, a
text encoding of `auto` (the default) implies a UTF-8 or ASCII
compatible source encoding. When a text encoding is otherwise specified,
it is unconditionally used for all files searched.

Since ripgrep's regex engine is fundamentally built on top of UTF-8,
this feature works by transcoding the files to be searched from their
source encoding to UTF-8. This transcoding only happens when:

1. `auto` is specified and a non-UTF-8 encoding is detected.
2. A specific encoding is given by end users (including UTF-8).

When transcoding occurs, errors are handled by automatically inserting
the Unicode replacement character. In this case, ripgrep's output is
guaranteed to be valid UTF-8 (excluding non-UTF-8 file paths, if they
are printed).

In all other cases, the source text is searched directly, which implies
an assumption that it is at least ASCII compatible, but where UTF-8 is
most useful. In this scenario, encoding errors are not detected. In this
case, ripgrep's output will match the input exactly, byte-for-byte.

This design may not be optimal in all cases, but it has some advantages:

1. In the happy path ("UTF-8 everywhere") remains happy. I have not been
   able to witness any performance regressions.
2. In the non-UTF-8 path, implementation complexity is kept relatively
   low. The cost here is transcoding itself. A potentially superior
   implementation might build decoding of any encoding into the regex
   engine itself. In particular, the fundamental problem with
   transcoding everything first is that literal optimizations are nearly
   negated.

Future work should entail improving the user experience. For example, we
might want to auto-detect more text encodings. A more elaborate UX
experience might permit end users to specify multiple text encodings,
although this seems hard to pull off in an ergonomic way.

Fixes #1

1 files changed, 6 insertions, 8 deletions

diff --git a/README.md b/README.md
index 6827e703..cc5923ea 100644
--- a/README.md
+++ b/README.md
@@ -83,6 +83,10 @@ increases the times to `3.081s` for ripgrep and `11.403s` for GNU grep.
   of search results, searching multiple patterns, highlighting matches with
   color and full Unicode support. Unlike GNU grep, `ripgrep` stays fast while
   supporting Unicode (which is always on).
+* `ripgrep` supports searching files in text encodings other than UTF-8, such
+  as UTF-16, latin-1, GBK, EUC-JP, Shift_JIS and more. (Some support for
+  automatically detecting UTF-16 is provided. Other text encodings must be
+  specifically specified with the `-E/--encoding` flag.)
 
 In other words, use `ripgrep` if you like speed, filtering by default, fewer
 bugs and Unicode support.
@@ -101,18 +105,12 @@ give you a glimpse at some important downsides or missing features of
   support for Unicode categories (e.g., `\p{Sc}` to match currency symbols or
   `\p{Lu}` to match any uppercase letter). (Fancier regexes will never be
   supported.)
-* If you need to search files with text encodings other than UTF-8 (like
-  UTF-16), then `ripgrep` won't work. `ripgrep` will still work on ASCII
-  compatible encodings like latin1 or otherwise partially valid UTF-8.
-  `ripgrep` *can* search for arbitrary bytes though, which might work in
-  a pinch. (Likely to be supported in the future.)
 * `ripgrep` doesn't yet support searching compressed files. (Likely to be
   supported in the future.)
 * `ripgrep` doesn't have multiline search. (Unlikely to ever be supported.)
 
-In other words, if you like fancy regexes, non-UTF-8 character encodings,
-searching compressed files or multiline search, then `ripgrep` may not quite
-meet your needs (yet).
+In other words, if you like fancy regexes, searching compressed files or
+multiline search, then `ripgrep` may not quite meet your needs (yet).
 
 ### Is it really faster than everything else?