Removing old website generation
`make test` should build direnv first
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.
Stdlib documentation
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.
stdlib: layout go
Proper file touching

The old version was truncating the target file...
More stdlib doc
Make the `direnv stdlib` command public

This allows to easily see what commands are available in the stdlib.
Change the `direnv edit` logic a bit.

By default we don't create a new .envrc is no path is provided. This allows
the command to be less ambiguous.
New failing testcase: the .envrc execution context has the symlink resolved
Didn't find a way to fix that last bug :/ Marking as known issue for now.
Reworking the doc

Removed most of the manpages, they're too much work to keep in sync.

Separated the main manpage with the README.md. It was a nice idea but the
format it not the same.