Add an option for rich selection queries using and/or/not syntax

[mjambon]

e.g. ./test run --tag 'slow and not todo' where slow and todo are user-defined tags that some tests carry.

See also #4.

[mjambon]

We might also want to mix tags with text search. For example, I can imagine myself using this:

./test -q '(foo or "blah blah") and tag:lang and not tag:todo'

and could be omitted as in old web search engines:

./test -q '(foo or "blah blah") tag:lang not tag:todo'

I think I prefer and to be explicit.