Livereload

This PR introduces live-reloading in gitbook. When running 'gitbook serve', a tiny-lr server is started. As soon as the gitbook process restarts its 'normal' webserver (because of a file change), the tiny-lr server triggers a browser refresh.

Important: http://livereload.com/ needs to be installed in the browser to make the refreshing work. The livereload.js script import is not injected into index.html.

To make the livereload work smoother (to avoid "double refreshes"), I additionally introduced a option to disable the cache manifest generation.