Further NPM support for the new :bundle target

Got extra-mains and auto-testing up and running for the new bundle target along with an
intial docs page https://figwheel.org/docs/npm.html to help get
started with an NPM based ClojureScript project.

• Added templating to :bundle-cmd cljs option: it interpolates :output-to and :final-output-to
• :bundle-cmd only bundles one time when watching and compiling
• set the fighweel option :bundle-once to false to have bundling occur on every compile
• set the fighweel option :final-output-to if you don't want the default of :output-to + "_bundle"
• logs the bundle command when it runs for easier debugging