Added --format to rustc

Usage: rustc filename.rs --format
It formats the the specified file with --pretty normal.

There are two problems with this PR.

1.) I am using "process_output" instead of hooking up in pretty_print directly. I didn't want to step on someones toes.

2.) Pretty print doesn't do a partial format. This mean that the source file doesn't get formated if the source code contains syntax errors.


I am currently using this with sublime. Every time I press ctrl+s it automatically saves and formats my current file.