Have `rustfmt` group `use` statements

[brettcannon]

many people prefer to format their use lines in three blocks: imports from the standard library, imports from external crates, and then imports from modules within the same project. rustfmt supports this via the option group_imports = StdExternalCrate

[sanders41]

To use this you need to switch to nightly as it is not stable yet (reference here). Using it on stable throws an error:

Warning: can't set `group_imports = StdExternalCrate`, unstable features are only available in nightly channel.

[brettcannon]

Darn. I'll leave this open for when it reaches stable. Thanks for leaving a comment about the availability!