Populate .vimrc.before with all spf13 options

* .vimrc.before should not be modified (just like any other non-local
  or non-fork config file)
* .vimrc.before contains a list of all options that can be set
* .vimrc.before.local is where these options would actually be set
* support .vimrc.before and .vimrc.before.fork in install scripts