Add per-file overrides using field lists

[ItayZiv]

Use field lists to implement per page overrides
Closes #11.
It seems like __init__.py might be due for a slight rewrite after this change.

[ItayZiv]

Probably should be done at this point, but it really does seem like __init__.py is getting a bit messy, and test_options.py might also be due for a cleanup to make all the tests more similar.

[TheTripleV]

Thanks for this PR. It is much needed. I didn't notice it was marked as ready till now, sorry!

The PR seems fine logic wise, there's just a few changes I want to increase readability of the code.

[TheTripleV]

Regarding the names of the fields, do field lists support underscores? If so, I think using underscores instead of hyphens would be better as the variable names in conf.py and the names of the field list keys would match.

Also, since making this extension support all opengraph tags would be nice. So, transforming all field list entries of the form
:og:X: Y to tags mapping og:X to Y.

[ItayZiv]

Regarding the underscores for names, I think it's supported but field lists everywhere else use hyphens, so I think it'd be better to be consistent with that.

Stale

[TheTripleV]

Note, relative image paths in the custom overrides are broken (intentionally) in this PR. A fix should be included as a part of #53.