Accept callable as patch_generator (#235)

[dholth]

The patch system now accepts a function (callable) in addition to a file path or conda package path. The callable should accept (repodata, subdir) arguments and return patch instructions.

Description

Fix #235

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[danyeaw]

Thanks for this @dholth! Since #235 called out easier testing as the motivation, could what do you think about adding a small test that passes a callable directly?

[danyeaw]

I think we need an if not callable guard here.