Add various comments
Start separating rules from upadesha.py
Refactor some code in filters.py
Add support for rule 6.1.45
Add miscellaneous comments
Refactor 'tasya' code

All of it now lives in an operator.
Refactor filter code

Parameterized filters are now specified by class instances. The code
is much easier to understand and a lot more flexible.

Closure-style filters have been removed for the time being, but they
might be introduced in the future. The non-closure implementation is
not substantially slower.
Add Sphinx starter code