WIP: Add substitution definition code and tests

[melissawm]

It took me long enough because I think I was trying to do too much. This is a first pass and definitely needs improvement but it does work. I need some feedback on whether this is the right approach and whether we should look at resolving external links too?

TODO:

• [ ] Add tests for substitution definitions in ascii_expected (currently failing)
• [ ] Implement substitution definitions for all section types in gen.py (including narrative and module docstrings)
• [ ] Implement support for custom inline directives

[Carreau]

Ok, this is a bit more complicated that I thought, I've push a few commits to try to see how we can push this a bit further.

[Carreau]

This should be almost all good with the exception of numpy narrative that makes use of a global substitution using |version|, I guess we can either skip that for now or implement a global substitution.