Allow task tags to start with a special sign

The implementation of `CombinedWordRule` has changed insofar that a word
that is already read but not matched can now be rescanned by other
rules. Previously the word was associated with a default token.

Fixes #1000991
Merge pull request #633 from sschaef/t1000991-task-tags-with-special-start-sign

Allow task tags to start with a special sign