Clarify requirements regarding lxml (fixes #70)

[ci skip]
Add xkcd plugin
Rewrite the tests to not use an external source of data
Fix the tests so they don't freeze everything ever (Thanks Alan!)
Refactor the main function slightly
Also convert html escape characters
Also mock the 404 page
Actually properly mock the 404 page
Remember to also match against empty strings
Merge remote-tracking branch 'LordAro/xkcd'
Enable xkcd plugin
Add xkcd linkinfo integration
A few style cleanups, add some linkinfo integration tests
Restructure unit tests
Add helpers for fixture files, rewrite test_plugin_xkcd to use them
Fix a comment

[skip ci]