filter out files that are not baileyjs in --watch

This prevents a endless loop of:
```
src/app.js changed, recompiling...
-----------
src/app.bs -> src/app.js
-----------
Done! Looking for more changes...

src/app.js changed, recompiling...
-----------
src/app.bs -> src/app.js
-----------
Done! Looking for more changes...
```