corpkit: Classes, methods, improved concordancing

This release marks a transition to a class-and-method structure, rather than a collection of functions. Users now instantiate a Corpus object with methods for parsing, interrogating and concordancing. Interrogations output Interrogation objects, which have methods for editing, plotting, saving, etc.

Another major update is that the concordance() method takes the same core arguments as the interrogate() method. This means that users can quickly check that their interrogation is counting what they think it is.

There have also been some bugfixes, documentation updates, and that kind of usual stuff.