Add Link header support

[ashlkv]

Fixes #27

• Feed loaders now extract feed urls of their respective types from Link header using the shared utility function
• Added and updated the tests

Motivation

The Link http header can sometimes contain the link to the feed. This http header is used when the document itself is difficult to modify.

• Feed loaders look for a feed url of their respective type in the Link http header
• If found, this link or links take priority over links found in the document's html
• Works even if the document is not html (should it?)

[ai]

Very good comment. But we don’t use it in another functions.

Can we keep the old comment-less format in PR?

But I am open to discuss adding more comments (just prefer to have some policy).

[ashlkv]

Let's consider adding more comments. It's hard to understand what functions do otherwise, even looking at tests. Especially for newcomers. And especially now without ui.

[ai]

Thanks a lot! Very professional work.