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

src/app.rs

@@ -22,6 +22,11 @@ Note that ripgrep may abort unexpectedly when using default settings if it
 searches a file that is simultaneously truncated. This behavior can be avoided
 by passing the --no-mmap flag.
 
+ripgrep supports configuration files. Set RIPGREP_CONFIG_PATH to a
+configuration file. The file can specify one shell argument per line. Lines
+starting with '#' are ignored. For more details, see the man page or the
+README.
+
 Project home page: https://github.com/BurntSushi/ripgrep
 
 Use -h for short descriptions and --help for more details.";
@@ -513,6 +518,7 @@ fn all_args_and_flags() -> Vec<RGArg> {
     flag_max_filesize(&mut args);
     flag_maxdepth(&mut args);
     flag_mmap(&mut args);
+    flag_no_config(&mut args);
     flag_no_ignore(&mut args);
     flag_no_ignore_parent(&mut args);
     flag_no_ignore_vcs(&mut args);
@@ -1113,6 +1119,20 @@ This flag overrides --mmap.
     args.push(arg);
 }
 
+fn flag_no_config(args: &mut Vec<RGArg>) {
+    const SHORT: &str = "Never read configuration files.";
+    const LONG: &str = long!("\
+Never read configuration files. When this flag is present, ripgrep will not
+respect the RIPGREP_CONFIG_PATH environment variable.
+
+If ripgrep ever grows a feature to automatically read configuration files in
+pre-defined locations, then this flag will also disable that behavior as well.
+");
+    let arg = RGArg::switch("no-config")
+        .help(SHORT).long_help(LONG);
+    args.push(arg);
+}
+
 fn flag_no_ignore(args: &mut Vec<RGArg>) {
     const SHORT: &str = "Don't respect ignore files.";
     const LONG: &str = long!("\
@@ -1182,8 +1202,7 @@ part on a separate output line.
 }
 
 fn flag_path_separator(args: &mut Vec<RGArg>) {
-    const SHORT: &str =
-        "Set the path separator to use when printing file paths.";
+    const SHORT: &str = "Set the path separator.";
     const LONG: &str = long!("\
 Set the path separator to use when printing file paths. This defaults to your
 platform's path separator, which is / on Unix and \\ on Windows. This flag is