Emit finish event when done

Task is setting `browserify.options.external` configuration, which is pretty neat, however if I need some other externals there, things get rather complicated. I am using it like this:

	grunt.registerTask 'dev', ->
		externalsOption = 'browserify.options.external'
		externals = grunt.config externalsOption

		grunt.event.once 'browserifyBower:finish', ->
			bowerExternals = grunt.config externalsOption
			grunt.config externalsOption, bowerExternals.concat externals
			grunt.task.run 'browserify:signin', 'browserify:main'
		
		grunt.task.run 'browserifyBower:vendor:nowrite'

Maybe there is better way, but this just works for me.