Related to #85

Everything should now be configured at runtime, with the exception of
the `Pleroma.HTML` scrubbers (the scrubbers used can be
changed at runtime, but their configuration is compile-time) because
it's building a module with a macro.

Gosh please don’t break ourselves…

Also this is copy-paste of the list in lib/pleroma/formatter.ex,
I think this should be put in a common variable, but where?