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

tests/tests.rs

@@ -1711,6 +1711,22 @@ fn compressed_failing_gzip() {
     assert_eq!(err.contains("not in gzip format"), true);
 }
 
+sherlock!(feature_196_persistent_config, "sherlock",
+|wd: WorkDir, mut cmd: Command| {
+    // Make sure we get no matches by default.
+    wd.assert_err(&mut cmd);
+
+    // Now add our config file, and make sure it impacts ripgrep.
+    wd.create(".ripgreprc", "--ignore-case");
+    cmd.env("RIPGREP_CONFIG_PATH", ".ripgreprc");
+    let lines: String = wd.stdout(&mut cmd);
+    let expected = "\
+For the Doctor Watsons of this world, as opposed to the Sherlock
+be, to a very large extent, the result of luck. Sherlock Holmes
+";
+    assert_eq!(lines, expected);
+});
+
 #[test]
 fn feature_740_passthru() {
     let wd = WorkDir::new("feature_740");