Feature request: Type coverage for flow

It'd be great if ALE could indicate where flow has not inferred a type. This would help get to 100% type coverage.
Flow has great type inference, meaning that we can keep explicit annotations to a minimum.
But it'd be nice to see where it doesn't quite figure something out *in the editor*.

Currently, I use `flow-coverage-report` to find which files have uncovered expressions, and then `flow coverage --color $FILENAME` to see which expressions are uncovered. It's cumbersome.

There's already a vim plugin that does this: https://github.com/carlosrocha/vim-flow-plus, but I'd rather have ALE do it and not have to add yet another plugin.