Allow specifying command-line parameters in noxfile

[theacodes]

Related #133, #40, #135.

Noxfiles should be able to specify some of the command-line args in the noxfile, for example:

import nox

nox.options.envdir = ".cache"
nox.options.sessions = ["tests", "lint"]

...

Command-line args should override these "defaults" in the noxfile.

[moshez]

I'm happy to take this on. I've been looking for an excuse to learn about nox internals for a while :)

[theacodes]

@moshez go for it! holler if you run into any trouble or have any questions.

[theacodes]

The exact list of flags that should be specifiable in the noxfile are:

  --envdir ENVDIR       Directory where nox will store virtualenvs.
  -s [SESSIONS [SESSIONS ...]], -e [SESSIONS [SESSIONS ...]], --sessions [SESSIONS [SESSIONS ...]]
                        Which sessions to run, by default, all sessions will
                        run.
  -k KEYWORDS, --keywords KEYWORDS
                        Only run sessions that match the given expression.
  -r, --reuse-existing-virtualenvs
  -x, --stop-on-first-error
  --error-on-missing-interpreters
  --report REPORT
  --nocolor
  --forcecolor

Basically, everything but --noxfile can be specified in the noxfile.