Summary:
Added a watch mode to the bundle command of flipper plugins. This makes it easy to test develop plugins locally, even when you are using a production build of flipper, which lowers the barrier of developing / fixing plugins.
opted for `fs.watch` over watchman to avoid some overengineering:
1. pkg / pkg-lib don't have the watchman utilities that would be needed for this. I wasn't sure if I could move those over from `static` without breaking the bootstrapping process
2. watchman is often a nuisance on non FB machines that aren't set up for it. fs.watch in contrast doesn't have any further dependencies or setup requirements, and is much more likely to work ootb.
3. since we watch only the `src` folder we don't really need the watchman optimizations. (so for a package.json change people would have to restart, but I don't think that is much of a problem).
Reviewed By: jknoxville
Differential Revision: D21523814
fbshipit-source-id: b1de72b7d01c6fc50cb8ce5709f54f8019eb89e4
080085e788