Added parameters for pandoc converter.

Could be made more generic if it turns out you want to apply different
templates to other converters later, but works for now...
Tweaks for work-making.

The builders code could probably still use some tidying to look less
kludgy, but it works.

Now each theme can have a customisation overlay, although defining
things that conflict with already defined styles can result in
excitingly difficult to predict behaviour.
Merge pull request #1 from ikirker/master

PR to allow slides to specify a style, extra CSS files to copy, and a syntax highlighting colour scheme.