Add support for `:maintenance_dirname`

This adds support for customizing the directory the maintenance.html gets uploaded it. I found this necessary to add for [moonshine](https://github.com/railsmachine/moonshine), because for a few reasons, we keep 'system' in `#{shared_path}/public/system` instead of `#{shared_path}/system`.

I also added documentation for this and the other configuration options.

I expect to be using this gem more soon, so if you need help with maintenance, please let me know!