Switch plugin managment to dein

[yarbelk]

This is almost complete (dein branch):

• install/manage plugins with dein
• update plugins automatically once they've been installed
• Rake task correctly installs dein when run
• All plugins have their dependencies either installed automatically, or have instructions in
  the readme

[snuggs]

The work looks AMAZING!!! Lookin' good neons ;-) Let's make this a WIP Pull Request yes?