Add types to autogenerated schema docs

[lucacorti]

Hi, this adds options types to the documentation generated with NimbleOptions.docs/2.

[whatyouhide]

I like this idea. However, I think:

1. We should not show anything when the type is {:custom, ...}. Let's let the user write it down in the docs.
2. I don't think we should put backticks around types. I think a good output is :some_option (integer) - ....
3. You moved the "required" part alongside the type. Let's not do that. A key can be required even if :type is not in the schema. Let's revert that change.

[lucacorti]

Formatting should now be done in the key (type) and optionally - (Required.) Documentation format.

[lucacorti]

Also caught a bug with {:list, subtype}, fixed and added it to the tests. Anything else we should fix?

[lucacorti]

Last round of changes should be complete.

[whatyouhide]

Thanks @lucacorti! 💟