Add a "filter" options.
Add test for filter option.
Parse the filter using the standard ast module.
First implementation of the filter, using a visitor.
Enable full filter test.
Fix a bug.
Remove stray debug line.
Document filtering implementation and simplify a few details.
A few more minor documentation updates.
Update changelog.
Coverage for option clash warnings.
Coverage of more operators.
Test the filters just added.
Fix pragma.
Change assert into value error (it checks user input).
Properly report syntax errors in filter expression.
Test more invalid things.
Bugfix, and recover from more syntax errors.
Count all syntax errors to ensure they are all triggered in the test.
Delete undocumented behaviour.