Use watchify, reuse bundles

This speeds up builds after the first one by a factor of 100×:

```
INFO [framework.browserify]: Browserified in 50309ms, 2172kB
 ⋮ snip
INFO [framework.browserify]: Browserified in 582ms, 2172kB
```

n.b. that this won't allow for adding new files while watching. Is this a showstopper?