Adds isolated visual tests for javascript plugins (decoupled from docs)....

... This makes it faster and easier to test/develop js functionality not represented in unit tests, and gives us a playground for interactions, etc.

It also makes it so developing javascript is now decoupled form jekyll, which should make everything faster and less painful.

This commit also reverts my filter commit https://github.com/twbs/bootstrap/commit/9900771aa7f1b3ddcee49aec84082104776ace70 which broke scrollspy for dropdowns