Mention :Rset root_url under :Rserver

Should make searching for the replacement easier
Don't need to create tmp anymore
Allow g:rails_ctags_arguments to be an array
Support path_additions globally
Rewrite explanation of global settings

All the "seldom used" options have been cleaned up.
Add a little breathing room
Provide g:rails_projections

I have been dragging my feet about this because I want to steer people
towards config/editor.json, but seeing as how everything else has a
global, denying this one would just be stubbornness.
Roll g:rails_abbreviations into plugin
Move default config docs next to editor.json docs
Fully document projections