PERF: Concatenate scripts in development mode

This improves my DOMContentLoaded from 9s+ to less than 4s.
Pinging @SamSaffron on this because this was previously controversial.

This implementation adds the `@sourceURL` directive so chrome correctly
identifies the source files.

lib/es6_module_transpiler/tilt/es6_module_transpiler_template.rb

@@ -127,6 +127,7 @@ module Tilt
         req_path = "/assets/#{scope.logical_path}.#{extension}"
 
         @output << "\nwindow.__jshintSrc = window.__jshintSrc || {}; window.__jshintSrc['#{req_path}'] = #{data.to_json};\n"
+        @output << "\n//@ sourceURL=/#{scope.logical_path}\n"
       end
 
       @output