Smart Operators for R language

[walmes]

Dear William Xu (@xwl),

In this pull request I send you the contributions made to adapt the electric-spacing for the R language. I preferred to create a new file, electric-spacing-r, to implement the electric rules between operators. R users need only electric-spacing-r and can enable it with

(require 'electric-spacing-r)
(add-hook 'ess-mode-hook #'electric-spacing-mode)

[xwl]

Hmm, i think a cleaner solution is that electric-spacing-r.el still (require 'electric-spacing) as base, then apply necessary changes on top of affected functions (e.g., using advices).

Ultimately, I should add some sort of plugin support, making it easier to override for each language..