Document :reuse in the sources, too
Introduce new option, make use of it, fix docstrings
Add comments
Rewrite if-cascade to cond form
Rename default to fallback rules

There is otherwise a subtle clash between defaults as things to be
unconditionally done and the rule to be used when nothing else matches.
Hence the renaming to "fallback" to make clear it is in no way a
default, just another option to use when nothing else works out.
Bump version
Correct misleading error in description