Add derive feature that reexports rustyline-derive's API

[gwenn]

Check:

• [ ] cargo doc
• [ ] doc.rs features
• [ ] test/example

[figsoda]

cargo doc

not sure what documentation to add since there wasn't any for rustyline-derive, but it does show up in the Derive macros section on docs.rs

[gwenn]

cargo doc

not sure what documentation to add since there wasn't any for rustyline-derive, but it does show up in the Derive macros section on docs.rs

I am not sure if it is possible but ideally they should be marked as available only with derive feature like here.

[figsoda]

that seems to be possible, I have added that

[gwenn]

Thanks !