Turn mdsplit into a Python library

[fbuchinger]

The CLI interface of mdsplit is really neat, but for some use cases I'd really need a python library version of it:

• rewrite relative image links in the split out markdown files so that they work in the new directory structure
• clean up markdown text (background: I am working with many markdown files that have been converted from .docx via pandoc)
• decide whether a split markdown file should be written out or merged with the next document (think of a h1-level headline immediately followed by a h2-level headline, currently the h1 headline would be put into a document of its own)

Thus the library version needs to represent the split markdown documents as objects and offer methods to access their contents as well as some merge logic.

[markusstraub]

I'm happy to merge pull requests but unfortunately don't have time to look into this.