Add -p/--projects option.

The code still defaults to looking at your watched projects configured
in gerrit.  This option allows you to specify a different list of
projects to consider, instead.

I am set up to watch most projects in gerrit, but when looking for a
review, I usually only want to consider the projects for which I'm a
-core team member.  So, now if I only want to see nova changes, I can do
something like:

  $ next-review --username russellb \
  > --projects openstack/nova,openstack/python-novaclient