Add generated css and js to gitignore

I found it nice to ignore these files, they are automatically generated anyway (i.e. reveal.min.js always has the date of minify-ing, which I don't want to commit all the time). Should just track the source unless there is a good reason to hand-curate these versions.
next release will be 3.0.0
Merge branch 'patch-1' of https://github.com/jamestwebber/reveal.js into dev
stop tracking minified files #783