Implement cljs.closure/watch logic for rebuilds

[Deraen]

ClojureScript watch no longer (since Feb 2015, after Boot-cljs was originally written) calls build each time any file is changed. Only if .cljs, .cljc, .clj or .js file is changed, compiler is called.

https://github.com/clojure/clojurescript/blob/master/src/main/clojure/cljs/closure.clj#L3096-L3100
clojure/clojurescript@0db90e6

Boot-cljs should copy this behavior, to optimize cases where only .css, .less etc. file is changed.