Script update, bash completion

* use subprocess instead of deprecated popen
* use os.chdir() with contextmanager instead of direct 'cd'
 * http://docs.python.org/2/library/contextlib.html#contextlib.contextmanager
* usage formatting to be consistent with other git commands
* better color support
* enable `bash completion` for `git pluck` that is identical to the standard `git` completion with the addition of completing `--nogit`