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

doc/rg.1.md

@@ -125,6 +125,13 @@ Project home page: https://github.com/BurntSushi/ripgrep
 : Only show matches surrounded by line boundaries. This is equivalent to
   putting ^...$ around the search pattern.
 
+-z, --search-zip
+: Search in compressed files. Currently gz, bz2, xz and lzma
+  formats are supported.
+
+    Note that ripgrep expects to find the decompression binaries for the
+    respective formats in your system's PATH for use with this flag.
+
 # LESS COMMON OPTIONS
 
 -A, --after-context *NUM*