Added --db-filter and --user-filter flags

[kmoppel]

Flags for displaying processes on only desired DBs or users. Input should be Postgres regexes.

[kmoppel]

Useful for test instances with lots of DBs for example. Pretty straightforward patch, security (relying on psycopg2 parameter parsing) shouldn't also be a problem as user anyways has access to the DB for normal queries.

[slitsche]

Hey Kaarel,
Travis CI build failed. Could you please have a look at it?

[kmoppel]

Hey Stefan! Ah yes, the linter. Hmm, but seems the failing code lines are actually the old one. And after running "autopep8 -i pg_view.py" I got around 30 other formatting changes..so not sure if I should commit it? Maybe I'm using the wrong tool?

pg_view.py:146:1: E305 expected 2 blank lines after class or function definition, found 1
pg_view.py:3728:1: E305 expected 2 blank lines after class or function definition, found 1
setup.py:20:1: E305 expected 2 blank lines after class or function definition, found 1

[a1exsh]

@kmoppel I find it that often than not that flake8 adds more complaints than it was reporting before. I think we should open a separate PR, then you could update your branch to the recent master.