Feature request: F#

For the sake of Elm development, I switched from Syntastic to ALE, on the recommendation of the Elm plugin, and the fact that Syntastic wasn't working with it. Now my F# stuff doesn't work so I"m going to have to toggle back and forth between ALE and Syntastic depending on what I'm working on, which means using Syntastic most of the time, since I do a lot more F# than Elm.

So it would be nice if ALE played with [this](https://github.com/fsharp/vim-fsharp). Should I also open an issue on that repo?