Add negation and disjunction to query options

[charle36]

Negation works in some cases, but not in others. If you write "not due", the "not" part is ignored. Moreover, if you write "heading does not include" you reject any task without heading.

There are no option for an "OR" operator in the queries. We can get around the issue most of the time, but that operator would simplify some operations.

[schemar]

Thank you for the input. There are some discussions around queries.

Please see, for example, #121 for filter operators (OR).

[charle36]

Thanks! Your plugin is awesome!