Split clang option arguments using POSIX standard.

Uses Python standard library's `shlex` class to provide
accurate option parsing. It also prevents options quoted using
single quotes from being mishandled. i.e. The quotes are stripped
as expected.

Escaping using the escape character `\` is not supported at the
moment due to the clang_complete.vim converting all backslashes to
forward slashes (which is incorrect behaviour I might add).
However this is consistent with the current behaviour regardless.