refactor rollup usage to utilize rollup watcher for watch command

[jonknapp]

Currently, the build command will kick off a single process that spawns new processes for each js file that needs to be processed. If there are multiple files, each is recreated whenever a javascript file changes during the --watch command.

If we refactor the js build command to kick off a rollup-watch or wrollup process per file, we can let the watch process monitor the sources files for recompilation. This is more performent and will make browsersync try to reload files only when there are changes that effect the source file.