Provide option to disable defaulting formatting provider

[cstrahan]

I like having the ability to manually run stylish-haskell from vscode, but it would be nice if I could use the brittany integration from the vscode-hie-server plugin for the default "format document" command. Would it be feasible to add an option to support this scenario?

There was some talk about similar things here: microsoft/vscode#11609

Thanks!