Implement a generic substitution mechanism about sbt-site

[jsuereth]

Note: Look into FMPP as a potential

[pvlugter]

We can look at a new sphinx directive in a plugin for better substitution for literal blocks. We should also check if this already fixed in sphinx itself.

[rkuhn]

I implemented pre-processing with configuration in SBT, see https://github.com/akka/akka/blob/4e4acef9592f3be911486e06ed22b0966740e554/project/Sphinx.scala#L66, now this functionality should be moved into the site plugin (I already made it somewhat configurable, but it has hard-coded location of copied tree relative to original doc source, to keep relative includes working).

[pvlugter]

Cool. I'll be moving everything needed for the akka docs into sbt-site shortly. I'll add the simple preprocessing, available to all the site generators.

[viktorklang]

Thanks mate!

[pvlugter]

Simple preprocessing added, based on Akka build.