config: add persistent configuration

This commit adds support for reading configuration files that change
ripgrep's default behavior. The format of the configuration file is an
"rc" style and is very simple. It is defined by two rules:

  1. Every line is a shell argument, after trimming ASCII whitespace.
  2. Lines starting with '#' (optionally preceded by any amount of
     ASCII whitespace) are ignored.

ripgrep will look for a single configuration file if and only if the
RIPGREP_CONFIG_PATH environment variable is set and is non-empty.
ripgrep will parse shell arguments from this file on startup and will
behave as if the arguments in this file were prepended to any explicit
arguments given to ripgrep on the command line.

For example, if your ripgreprc file contained a single line:

    --smart-case

then the following command

    RIPGREP_CONFIG_PATH=wherever/.ripgreprc rg foo

would behave identically to the following command

    rg --smart-case foo

This commit also adds a new flag, --no-config, that when present will
suppress any and all support for configuration. This includes any future
support for auto-loading configuration files from pre-determined paths
(which this commit does not add).

Conflicts between configuration files and explicit arguments are handled
exactly like conflicts in the same command line invocation. That is,
this command:

    RIPGREP_CONFIG_PATH=wherever/.ripgreprc rg foo --case-sensitive

is exactly equivalent to

    rg --smart-case foo --case-sensitive

in which case, the --case-sensitive flag would override the --smart-case
flag.

Closes #196

1 files changed, 44 insertions, 0 deletions

diff --git a/README.md b/README.md
index ac676863..502cfa5c 100644
--- a/README.md
+++ b/README.md
@@ -362,6 +362,50 @@ extensions.
 The syntax supported is
 [documented as part of Rust's regex library](https://doc.rust-lang.org/regex/regex/index.html#syntax).
 
+### Configuration files
+
+ripgrep supports reading configuration files that change ripgrep's default
+behavior. The format of the configuration file is an "rc" style and is very
+simple. It is defined by two rules:
+
+1. Every line is a shell argument, after trimming ASCII whitespace.
+2. Lines starting with '#' (optionally preceded by any amount of
+   ASCII whitespace) are ignored.
+
+ripgrep will look for a single configuration file if and only if the
+`RIPGREP_CONFIG_PATH` environment variable is set and is non-empty. ripgrep
+will parse shell arguments from this file on startup and will behave as if
+the arguments in this file were prepended to any explicit arguments given to
+ripgrep on the command line.
+
+For example, if your ripgreprc file contained a single line:
+
+    --smart-case
+
+then the following command
+
+    RIPGREP_CONFIG_PATH=wherever/.ripgreprc rg foo
+
+would behave identically to the following command
+
+    rg --smart-case foo
+
+ripgrep also provides a flag, --no-config, that when present will suppress
+any and all support for configuration. This includes any future support for
+auto-loading configuration files from pre-determined paths.
+
+Conflicts between configuration files and explicit arguments are handled
+exactly like conflicts in the same command line invocation. That is, this
+command:
+
+    RIPGREP_CONFIG_PATH=wherever/.ripgreprc rg foo --case-sensitive
+
+is exactly equivalent to
+
+    rg --smart-case foo --case-sensitive
+
+in which case, the --case-sensitive flag would override the --smart-case flag.
+
 ### Shell completions
 
 Shell completion files are included in the release tarball for Bash, Fish, Zsh