Investigate alternate frontends

[gchp]

Since the rustbox interaction was extracted to a frontend, it opens the door for implementing other frontends.

A possible frontend could be one using Conrod. This would be nice as it is also a pure rust implementation. Any other GUI would be cool though, as Conrod is pretty new and lacking some features we might need for this.

Frontends could be chosen at startup by a flag, or we could provide separate binaries for each frontend, like gvim and vim.