conditionally add arguments to installer OnionParsers based on user's local installer version?

Related to #7

Pip and conda both occasionally deprecate or drop support certain arguments and add new ones. On the one hand, it'd probably be nice in terms of "least astonishment" for the Onion parser to support all args that the user's local pip/conda executable supports. On the other hand, it could potentially hurt code portability if one of the installers decides to majorly change something at some point.

If we do want to do this, I think these changes are infrequent enough that we could just hard code everything and set up the parsers based on `pip.__version__`, etc. We could also parse the output of `pip install --help` to gather args and help messages on import, but that would be less efficient I think.