Use a crate attribute to load plugins

Use a crate attribute to load plugins

    #[plugin] #[no_link] extern crate bleh;

becomes a crate attribute

    #![plugin(bleh)]

The feature gate is still required.

It's almost never correct to link a plugin into the resulting library /
executable, because it will bring all of libsyntax and librustc with it.
However if you really want this behavior, you can get it with a separate
`extern crate` item in addition to the `plugin` attribute.

Fixes #21043.
Fixes #20769.

[breaking-change]