vim: add vim-mustache-handlebars submodle

This adds syntax highlighting for mustache and handlebars templates.
Replaces vim-handlebars, which is no longer maintained.
vim: remove deprecated vim-handlebars plugin
vim: remove unused vim-css-color plugin
vim: remove vim-numbertoggle

(vim-numbertoggle toggled between absolute and relative line
numbers. I'm removing it because it didn't actually work for
me.)
vim: fix submodule issues caused by mkrc

(submodules weren't added to .gitmodules)