Problem: Consider ways of removing repetition when calling amclient.py

[ross-spencer]

Considering this comment: #58 (comment) we might introduce a config file for amclient.py. Alternatives might be to supply values via environment variable. An example of the environment variable approach is twarc's approach to reading Twitter authentication keys: https://github.com/ruebot/twarc#install

[ross-spencer]

I notice as well that Fixity (AM's version) uses environment variables as well, so we have a precedent internally for doing this: https://github.com/artefactual/fixity/blob/master/docs/fixity.1.md