Add extension for variable substitution

[tbekolay]

One slightly annoying part of the Nengo Loihi docs is that we have to change several strings in the documentation when the NxSDK version changes. It would be nice if instead that version number could be part of the conf.py file, or some other location, and substituted in so that it's always updated everywhere, and updated easily.

ReST / Sphinx has the ability to replace text (see http://www.sphinx-doc.org/en/2.0/usage/restructuredtext/basics.html#substitutions) but this does not work inside code blocks. Here is a quick Sphinx extension to do it; we should add that extension (with a few minor tweaks) to Nengo Sphinx Theme and use it where appropriate.