Allow searching for tags #263

Steps to reproduce

1. Assign a tag called "Test" to a task not containing the word "Test"
2. Search for "Test"

Expected behaviour

Minimal

The task should appear.

Ideal

I also think there should be a way to search exclusively for tags. For instance "tag:Test" should only match tasks containing the tag "Test". A task called "Test" but without tag shouldn't be in the search results.

Actual behaviour

The task does not appear (does not match the search).