Change `rbenv` to `use rbenv` in the stdlib
First pass at the prefix-loading idea as described in #40

Not sure if we should add everything if the directories don't exists.

On the other hand, we might want to have the vars set before the prefix
has been generated.
misc
More cleanup of the stdlib
Reworking the doc
Removing old website generation
Remove the set_use_prefix() command in the stdlib

There's no really good story for that. Homebrew is not really ment to have
multiple version of the same project (`brew cleanup` will remove older-ones)
and I don't know about other platforms.

It's better to create your own use_ extensions in the ~/.direnvrc for now.