Allow local overrides of rustfmt configuration

rustfmt configuration makes it wonderfully flexible, however, the configuration must be the same for the whole project. The are some settings that I would like to control on a per-file or even per-function basis. For example, match_arm_blocks -- most of the time I like it set to false, but some matches IMO look a lot better when it is set to true. (In fact they'd look even better if I could set match_arm_blocks to something like Force, where all arms in a match get braces.)

This is probably very non-trivial to implement, but I was imagining something like having an attribute to control rustfmt behavior locally:

#[rustfmt::configure(match_arm_blocks = true)]
fn foo() {
 ...
}

No files or tests are named. Start by investigating how rustfmt reads project-wide configuration and how Rust attributes could express local settings; then clarify the supported file and function scopes and configuration semantics. Done should include a workable local override for the match_arm_blocks example, with tests covering the behavior.