Add variable name lint

This lint complains about underscores and capital letters in variable
names.

Related Story:
  http://www.pivotaltracker.com/story/show/50417091
  ("Add lint for underscores in variable names, mixin names,
  placeholders, and function names")

Change-Id: Ia3d5bdc40b3a6b5a8b6b2b6fd4fdf9ffee731f0e
Reviewed-on: https://gerrit.causes.com/22801
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Adam Olsen <adam.olsen@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>