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.
For the sake of completeness, this works:

```
sys.path.insert(0, FilePath(__file__).parent().path)
from filters import IgnoreWordsFilter
from functools import partial
# The version can be similar to "0.1.2-99-ge08b10f", but is not a spelling                                                                             
# error                                                                                                                                                
spelling_filters = [partial(IgnoreWordsFilter, word_set=[version])]
del sys.path[0]
```

But again, the real win here would be to be able to throw away the extra module and the `sys.path` mangling - and this change doesn't accomplish that.