Add library support

[victornoel]

I would like to exploit holdup, but from some python code, is there anyway to make it more like a library on top of the CLI?

[ionelmc]

It needs some refactoring to have a holdup.checks module with all the classes and a run function that does the running (needs to be extracted from holdup.cli.main). You interested in doing a PR?

[victornoel]

@ionelmc I won't be able to, no, sorry