Version substitution 724

Fixes #724

I have tested this by setting `version = '0.1.2-99-ge08b10f'` in `conf.py` and seeing an error:

``gettingstarted/tutorial/vagrant-setup.rst:202:ge:vagrant-setup``

without this fix, and no error with this fix.
You could still get rid of `IgnoreWordsFilterFactory` by using something pickleable in place of the `lambda ...`.  However, since you'll still need the separate filters module and you'll still have to mangle `sys.path` in awful fragile ways, there's probably no real benefit to doing this.