Turn the generate-filter.py script into a lektor plugin

With !24 (merged) merged, we have a python script to generate a CSS filter from a hex color code. The next step is to turn that into a lektor plugin, so users can just specify LEKTOR_ONION_LOCK_COLOR=abc instead of needing to run scripts/generate-filter.py abc => LEKTOR_ONION_LOCK_COLOR='filter: invert(98%) sepia(100%) saturate(7492%) hue-rotate(156deg) brightness(144%) contrast(59%);'

The script should be exposed as a jinja function (example), and the function should replace envvar('ONION_URL_LOCK_FILTER') in the templates.