Add type checking via flow

[ariamckinley]

Not sure if this is a road you want to go down @zsherman (if not feel free to close), but flow is a nice type checker that can be used on a file-by-file basis. So even if molecule ships with flow in it by default, the user doesn't have to continue using flow.

If this is something we'd like to do, I can get us setup with it. I have some experience working with flow, as we use it extensively in hedron.

[zsherman]

Big fan of this, let's do it.