Enumerate scripts in parallel

[soutaro]

Using parallel gem, ScriptEnumerator#each yields Script objects concurrently, in threads. You can pass --threads command line option to specify number of threads for check, console, and find commands.

If your config has preprocessor definition and your source files contain many preprocessed files, this patch improves the performance up to ~#of threads in your processor. I tested on my Core i5, 4 threads iMac, --threads=4 runs about three times faster than --threads=1 for HAML files.