Access to `optimist` used to generate help documentation correctly

I've written a plugin for nesh and it takes a command line option (`-f` or `--fibrous`). I'd like to use `optimist.describe()` to document what the option does so that users who run:
`nesh --help`
will see it listed there, but since when I `require('optimist')` within my plugin, I don't get the same optimist object, so when I call `.describe()` on it, it doesn't have any effect.

If you could expose that as `nesh.optimist` or something, that would make it easy to add help documentation.

If you're open to it, I can write a patch and submit a pull request when I have time.