Support hashing of bundles for cache breaking

Hi everyone,

So a very common set up is to hash bundles so that the client can cache them, but download new content whenever the bundle is updated ( and therefore will have a new hash ). However, we need to know the names of the files generated in order to serve them properly. I think I saw before that there was support for a hash() method on the "factory", but I may be mistaken.

In order to support this, I propose a hash option be added to a bundle config, with the type of hashing desired.

```
fuse.bundle("client/app")
.watch("client/**")
.hmr()
.hash("md5")
.instructions(" > client/index.ts");
```

then, we need the ability to get the names of bundles created. I was thinking as an async return from `run`, but if we're in watch-mode that may present some challenges, so I'm not sure the best way to retrieve the generated bundle names. Any input is welcome.

Thanks!