Enable exceptions to --global dns-provider

[cromulus]

The bulk of my apps reside on domains that I control, have access to, and can use dns authentication for letsencrypt certifications.

However, a small number do not, and while it's great to be able to set a global value, I would love to be able to make exceptions for individual dokku apps.

something like:

dokku letsencrypt:set [app] challenge-type http-01

or

dokku letsencrypt:set [app] dns-provider none

I defer to you all on how best to implement the user interface, but it would be tremendously helpful.

Thanks!