Make Rextract quotes configurable (so that one can use single quotes)

[jeromedalbert]

The rails-:Rextract doc says:

And you issue this command:

    :2,3Rextract post

Your file will change to this:

 1  <div>
 2    <%= render 'post' %>
 3  </div>

'post' uses single quotes, but if you try to reproduce the example and extract to a partial, vim-rails actually uses double quotes. Our team has the exact opposite convention as the one mentioned in #343, i.e. we use single quotes. Maybe we could make them configurable, or revert to single quotes?