Can't specify a custom commit message for native lockfile updates

[strugee]

Or at least, https://greenkeeper.io/docs.html#custom-commit-messages doesn't document how to do so.

Thanks for maintaining Greenkeeper <3

[JustinLivi]

Looks like there are a number of undocumented commit message config options: https://github.com/greenkeeperio/greenkeeper/blob/master/lib/default-commit-messages.js

[JustinLivi]

I'd be happy to tackle this issue, and open a PR with the relevant documentation changes if maintainers are interested 👍