New watch task

[mdo]

Adds an watch task to compile CSS while the local server is running. Likely need to expand to JS, or at least rename to watch-css if it only affects the CSS. To get the right output, I removed the docs/examples/ directory for the prefix task; it was rewriting all the example CSS and adding source maps for them (not needed).

[mdo]

Hmm, seeing Sauce fail on the grunt task:

> bootstrap@4.0.0-alpha.6 js-test-cloud /home/travis/build/twbs/bootstrap
> ruby -run -ehttpd . -p3000 > /dev/null & grunt saucelabs-qunit
sh: 1: grunt: not found

[Johann-S]

I added several commands

watch-css : which watch only css changes
watch-js: which watch only js changes
watch: which watch css and js changes

I hope it doesn't bother you I handled this @mdo 😟