Add support for auto-bash completion

[cmccandless]

See this gist.

Note: since by design argutil requires commandline.json, it may be simply or more accurate to build completion suggestions using that file instead of traversing the parser tree (as in above gist).

Suggested API for new functionality:

class ArgutilParser(argutil.ArgumentParser):
    def __init__(self, definition, *args, **kwargs):
        argutil.ArgumentParser.__init__(self, *args, **kwargs)
        self.definition = definition

    def suggest_next(self, argv):
        return ...  # List of valid arguments

## __build_parser__(...) should now return ArgutilParser instead of argutil.ArgumentParser