Feature Request: Add not filter

[adamanteye]

Filters can be combined with and or or. However, users may wish to exclude certain results from a search.

Suggested examples:

To show completed tasks that do not belong to project:misc:

task not project:misc completed

To further filter out tasks with 'H' priority:

task not project:misc and not priority:H completed

There may already be existing methods to achieve this, but I am currently unaware of them.