Add programatic access and documentation

[versae]

In some cases, like when running Averell as part of a bigger Python program, it would be very useful to be able to invoke the commands using a programmatic API. There might be one in place but it needs to be documented and as clean as possible. Something like an Averell class (class Averell) with a few methods:

averell = Averell(corpora_folder="...")
averell.corpora()  # list is a Python keyword
averell.download("adso")
averell.export("es", filename="es.json")
adso = averell.export("adso", raw=True)