Refactor image filter registration to use an attribute on registry instead of a setting key, and tracking a set of themes which filters are registered for, not just a single theme
Refactor request.image_url() to have a complete signature, and request.image_tag() to mirror it
Use 'from __future__...' statements universally for better forward compatibility
In config.add_theme() directive, don't defer the calling of other config directives: this will result in them not being committed
Add better __repr__ for Theme and FilterChain
Prevent image filter chains from being used outside of the correct theme context
Fix usage of config.add_theme() with a dotted string
Embrace argparse in pcompile command, clean up tests