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.
Search for executables using WhereIs

Not sure how shelling out is done, but otherwise can't find pandoc on my system. Maybe user vs
global install problem.
Adding possibility for separate html build directories
Add border inside slide
Merge pull request #2 from mdavezac/master

Style change + enhancements to build
Add booktabs needed for newer versions of pandoc.

See https://github.com/jgm/pandoc-templates/pull/48