Context filter added

Mostly cosmetic changes to emulate taskwarrior's contexts.
Added by Denis Zhuravlyov about 5 years ago

In new versions of TaskWarrior contexts were implemented. Contexts are permanent sets of user-defined filters which affects all operations and can be switched from one to another. To emulate such filtering I decided to add one more filter string with predefined tags to filter out some unneeded contexts. Maybe later I'll pick some particular settings out in separate config file.