Add --fail-fast option

It may not be desirable to continue running rbenv-all if the given command fails early. For instance, if one is running rspec and it fails with one version it would be desirable to halt immediately.

This pull request adds support for a `--fail-fast` option. When passed to rbenv-all it will check the exit value after each command was run and exit if the exit value is non-zero.

This pull request also reworks the argument parsing so that `--bare` and `--fail-fast` can be passed at once, in any order. It also adds help output.