Remove unused vim plugins

* haml, html and markdown highlight still work without these plugins
* treetop and textile are likely very situational
* unsure if anyone is using greplace
Remove old ignores

* sass gets compiled to `tmp/sass-cache` with Rails
* vendor/bundle_gems seems like an arbitrary vendor directory
* vim/.netrwhist is for a plugin we no longer use
List local files available to override
fix relative .aliases.local path
Add syntax highlighting to certain file types

* Syntax highlight `Appraisals` as Ruby. Used especially for our open
  source libraries.
* Syntax highlight rackup (`config.ru`) files as Ruby. Used in all our
  Ruby web apps. Rails apps are rarely touched but some of the Sinatra
  and Middleman apps are occasionally edited.
* Syntax highlight JSON files as JavaScript. Used in our Trail Map.