Add option to allow absolute links

[mfisher87]

Resolves #141

[mfisher87]

I'm realizing that even if we allow absolute links, the next step is to actually test the link. Absolute links would be tested against the root of the filesystem, and that's often I think not what users are going for.

To work around this we could also add an --check-links-absolute-link-base-dir=... option, assuming that absolute links are intended to be relative to some base dir. E.g. in a documentation generator, an absolute link is relative to the documentation root. That's my use case.

OR, we could add no additional functionality and use the existing ignore functionality to ignore links matching $\/.*. This means absolute links won't be checked -- in the case where the user is using some static site generator, that thing can be responsible for checking its own links. That works fine for me.

This PR is not working and ready, but I'd like opinions, so I'm marking it ready for review.