Added prefix input and tests

[atteggiani]

Fixes #3.

PR Overview

• A prefix input parameter was added, allowing the absolute link prefix to be easily controlled by the user. Default value has been set to current version's prefix site:.
• The prefix is escaped (using re.escape) to avoid potentially treating the prefix characters as regex special characters.
• A very simple unit testing suite was added, to increase the robustness of the plugin
• Few improvements in some portions of the plugin, without modifying functionality.

Integration tests

I didn't peform any integration test as I don't have quick ways to check that the outputs of the current version of the plugin (witihout this PR) are the same of the ones with this PR included (with default input parameters).
Please feel free to perform these tests, and if there are any issues I'm happy to work on them.

Cheers
Davide

[foosel]

This required some work which I took care of however:

• Your tests were broken - attempting to import the wrong plugin. I fixed that.
• pre-commit was not happy (which is why a check was red on your original PR), that's fixed too.
• I documented the new config option in the docs.
• I added the test suite to the CI build job.
• I changed some phrasing and reverted a change to the name of an internal function.