Support use of external template sources for website reporter

Hi @ananelson 

I wanted to be able to keep the website reporter template/scaffold files for our layout separately from the several dexy based repositories I have.

This commit would implement a "supplementary" reporter, with a path to a location with an additional dexy.yaml
The files from processing that folder are added to the current run, *at the reporter stage*, and templates are taken from that location.

I have not attempted to do the more complicated problem of merging two dexy.yaml files at the main run stage.

Let me know if you think this is useful/interesting.

Not ready for pull yet: I want to do as we discussed and set up some stuff for adding custom variables to the reporter which can be read by the templates, to tailor the templates.