Feature request: let the user set an arbitrary formatter

[treyburn]

It would be wonderful to let the user bring their own formatter.

In my particular use case, I want to use gofumpt over goimports but I can see also usage where a user wants to bring their own proprietary formatter.

It'd be great to see a --custom-formatter flag to reference the path to the formatters binary along with a --formatter-args flag to let the user pass in optional args for their customer formatter.