FR: Filter functions to emulate, write out rules coverage to file

Right now there is no way to do only start the emulator with a subset of functions enabled instead of all. It requires a custom build or filter the exports ourself.

It'd be great if the `firebase emulators:exec` and `firebase emulators:start` would accept an argument like `--functions a,b` or maybe the same syntax as `firebase deploy`: `--only functions:a,functions:b` (this is what I was expecting at first)