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, 1 insertions, 0 deletions

diff --git a/complete/_rg b/complete/_rg
index 6b62c169..1074597d 100644
--- a/complete/_rg
+++ b/complete/_rg
@@ -54,6 +54,7 @@ _rg() {
     '(--mmap --no-mmap)--mmap[search using memory maps when possible]'
     '(-H --with-filename --no-filename)--no-filename[suppress all file names]'
     "(-p --heading --pretty --vimgrep)--no-heading[don't group matches by file name]"
+    "--no-config[don't load configuration files]"
     "(--no-ignore-parent)--no-ignore[don't respect ignore files]"
     "--no-ignore-parent[don't respect ignore files in parent directories]"
     "--no-ignore-vcs[don't respect version control ignore files]"