Add feature gate derive for re-exporting proc macros to strum.

[oxalica]

This is a shortcut to use proc macros without manually specifying strum_macros as a dependency, just as serde and many other packages do. So we can simply use use strum::*; (or #[macro_use] extern crate strum; for rustc prior to 1.31.0) to derive.

It is already tested to be compatible with rustc 1.26.0

Maybe more documentation is needed?

[Dushistov]

This is great feature, now I have only

#[macro_use]
extern crate strum_macros;

on the top of my lib.rs/main.rs, all other extern crate I removed, thanks to 2018 edition.

@Peternator7 Is anything that should be fixed to make it possible to merge?

Oh, I missed that there is merge conflict, @uHOOCCOOHu can you resolve conflict or if you have no time I can submit separate PR with resolved merge issues?

[oxalica]

Rebased now

[Peternator7]

This is great!! Sorry for the delay getting to it, but it'll be a great feature.