feat: Relative Url

[whizsid]

Sorry for the delay. 😄 😆 Fixes #3

[whizsid]

https://github.com/hello-rust/lychee/pull/15/files#diff-9bfca03419915f4d4c69bb43d30e158a84c63d0f68c35c4991fd659d8b158f1cR10-R12

pulldown-cmark library is splitting http://[::1] into four parts (http://,[,::1,]) when the URL is not in a URL section.

[mre]

Looks great!
Only thing I'd add is a unit test for extract_links with a some base_url (on top of the integration test you already added). Other than that it's ready to merge from my perspective. Code is very readable.

[mre]

Thanks for adding the unit tests and for your contribution in general @whizsid. LGTM. 😊