Create ESS+ package

[jabranham]

As discussed in #732 many people want ESS plus some other things so we've agreed to provided ESS and ESS+.

I think we should remove julia-mode.el from ESS and include in ESS+

• julia-mode
• poly-R

and perhaps others? What do others think?

[jabranham]

Need to add documentation for this and #762 once it's done

[jabranham]

Would people prefer ESS+ to live here or at a new repo under the emacs-ess github organization?

Personally I'd prefer a new repo, since I don't really want ESS to take care of package management for me.

[lionel-]

I think a new repo makes sense to clearly separate the different projects.

[vspinu]

I actually think it might be better to have it here. People could download ESS and do make ess+ which would download necessary packages. This is not enough for a separate package IMO.