ISBN parser

Similarly to how entries can be added via DOI or arXiv Id, the same should be possible via ISBN.

To this extent, a variety of Web APIs exists which support querying bibtex data via ISBN, however, most of them require registration and/or authentication through API keys. I think this can pose a potential burden on the user unless cobib uses a single hard-coded API key, which I am not particularly in favor of, since this puts a lot of stress on the API in case many users start using this feature.

Instead, I would favor a key-less API. Querying the web a little bit I found the openlibrary.org API which matches these features. It supports ISBNs, LCCNs and OCLCs. It is also one API which is being used by isbnlib (a Python library in turn used by papis). However, I am reluctant to add another Python dependency for something as minor as an additional import parsing method.

Note: In case that we ever plan on expanding the functionality of the different addition methods we can come back to this option. This is likely going to happen when we start adding direct support for importing bibliographies from Mendeley, Zotera and the like because then I will probably add translator classes for each possible import rather than relying on simple methods within the parser module.