Organized CLI option parsing with node-optimist

I got tired of `ENOENT`, so I structured CLI parsing with node-optimist.

    $ cssmin --help
    
    /usr/local/share/npm/lib/node_modules/cssmin/bin/cssmin:13
                throw err;
                      ^
    Error: ENOENT, open '--help'

now looks like:

    $ cssmin --help
    Usage: node ./bin/cssmin [options] [<file>]
    
    If no file is specified, assume stdin for input.
    
    Options:
      --help, -h  Print usage info