Fix #1089 -- Make pip install nikola[extras] work.

Usage:

   pip install nikola                   # standard install
   pip install nikola[extras]           # w/ requirements-full.txt
   pip install nikola[full]             # alias for ↑
   pip install nikola[tests]            # w/ requirements-tests.txt SANS EXTRA
   pip install nikola[extras,tests]     # w/ requirements-tests.txt AND EXTRA

If installing locally (i.e. from a git clone), use:

    pip install .[extras]

and so on.