Allow compiler plugins to be loaded from the command line

[huonw]

Currently the only way to load plugins is by having #[phase(plugin)] extern crate foo; in the source, but one can easily have plugins (lints etc.) that make sense to only load occasionally or that make sense to load into external crates in an automated fashion, where editing the source is hard (E.g. my spellck lint).

This could be something like -Z plugin=spellck -Z plugin=grammarck etc. (-Z since presumably this will be unstable.)

[huonw]

cc @kmcallister

[kmcallister]

Seems reasonable. For hardening options I was considering a generic way to specify crate-level attributes at the command line. I wonder if the same mechanism could accommodate both.

[Manishearth]

@huonw Any pointers on where I should look?

Also, -Z is a bunch of flags right now (and can't take options), should I change the implementation to something else?

[kmcallister]

@Manishearth: Re-use the implementation of -C, which has more sophisticated parsing.

[Manishearth]

Alright, thanks!