Support for external js/css sources file + closure language_in compiler option

New configuration options: 

jsSourcesFile: a file containing new-line separated list of JS source files. Alternative for jsSourceFiles. 

cssSourcesFile: a file containing new-line separated list of CSS source files. Alternative for cssSourceFiles. 

closureLanguageIn: Google Closure language_in compiler option (of type enum LanguageMode).


External source files helps streamlining development: same list of files can be used to render individual <script> -tags for scripts so that there's no need to recompile the bundle each time something is changed - browser's reload is enough.