Support ebook format

[MCBoarder289]

The library checks are built to support the physical book search only (my own personal preference) by default.

Others have expressed the desire to find eBook availability. I can see this being useful, so here are some preliminary thoughts on what we need to consider:

• Updating all library parsers to search for ebooks
  • contract returns some availability string and url
  • Should we make the return string more structured? (Status codes so we can style the response, string that is a more detailed message, etc.) Would be a good time to refactor this.
• Add some kind of input to denote which format a user wants to search
• Are these the only two formats we want to support?

[MCBoarder289]

Addressed in https://github.com/MCBoarder289/shelf-help/pull/20