cli: allow multiple folder contexts

[cpyle0819]

Current behavior

ailly --context folder

Desired behavior (or equivalent)

ailly --context folder --folders path/to/folder path/to/another

Motivation

I have a folder full of project files, and another folder with tests for those files. I want to add both of those folders to my context so I can easily generate tests for my files or vice-versa.

[DavidSouther]

This is going to get unwieldy, fast... let's keep this as an idea for general "now that we know more, how do we actually want to handle context"