search: add support for searching compressed files

This commit adds opt-in support for searching compressed files during
recursive search. This behavior is only enabled when the
`-z/--search-zip` flag is passed to ripgrep. When enabled, a limited set
of common compression formats are recognized via file extension, and a
new process is spawned to perform the decompression. ripgrep then
searches the stdout of that spawned process.

Closes #539

src/main.rs

@@ -4,6 +4,7 @@ extern crate bytecount;
 extern crate clap;
 extern crate encoding_rs;
 extern crate env_logger;
+extern crate globset;
 extern crate grep;
 extern crate ignore;
 #[macro_use]
@@ -44,6 +45,7 @@ macro_rules! eprintln {
 mod app;
 mod args;
 mod decoder;
+mod decompressor;
 mod pathutil;
 mod printer;
 mod search_buffer;