Improve styling of multiple options

[jdkato]

For example,

message: Use "%s" instead of "%s"
swap:
  foo: bar or baz #  currently the only way to list multiple options

results in Use "bar or baz" instead of "foo". I think it would be better as something like:

foo: [bar, baz] # => Use "bar" or "baz" instead of "foo"

[jdkato]

I think I've changed my mind about this: with the current implementation you get the nicer formatting mentioned above (as done in c77c99b) without being restricted to an array.