Introducing the `direnv exec DIR COMMAND ...` command executor.

It could be useful to write little wrapper scripts or just make sure the
.envrc is loaded in a barebone context line in the init scripts.

Example:

    #!/bin/sh
    envrc_dir=`dirname $0`
    exec direnv exec "$envrc_dir" gem "$@"
Overload `direnv exec` to allow a command as it's first argument.