Add options for querying issues

[Diomidis Spinellis]

By default git issue foo would list open issues tagged, milestoned or assigned as foo. New options would allow more precise selection.

• -A assignee
• -T tag
• -M milestone
• -c (closed issues)
• -a (all — open and closed — issues)
• Multiple selection options would be anded together

See also issue #42.

[Diomidis Spinellis]

WE now support -a and queries for a tag or milestone. Implemented by @vyrondrosos as part of GSoC 2019.