treefmt cli: allow explicitly specifying a specific formatter

[flokli]

Sometimes treefmt with a bunch of formatters is a bit slow. In some cases, I'd like to explicitly narrow down the list of formatters to run.

It'd be nice if I could only run a subset of formatters, by invoking treefmt -f nix,terraform (or similar syntax)

[zimbatm]

I like this. :+1:

[brianmcgee]

@zimbatm should this issue be closed as it looks like it was already merged?

[zimbatm]

thanks