Add process method and 'include_path' options

[markstos]

Michael,

Here's a patch for the 'process() and 'include_path' features. Docs and tests are included. I also looked at adding something like 'pre_define' or 'constants', but have decided to defer that for now.

Mark

[mpeters]

Mark, sorry I took so long to get this. At first I didn't want this feature as I just saw it as needlessly complicating things, but now that I've had some time and re-read it I think it would work.

Can you please rebase your branch against master and fix any conflicts (I think mainly just the difference in the change log file). And then I'll re-review and merge.