Add unit tests

[timvink]

Enabling some CI with unittests can improve the plugin quality.

mkdocs search plugin has some nice examples of how this can be done:
https://github.com/mkdocs/mkdocs/blob/master/mkdocs/tests/search_tests.py

[timvink]

Added unittests, CI with github actions and a coverage badge