+ Adds a tag `repeated` which allows `-v -v -v`, `-vvv`, or `-v=5` to
  set an `int` field.
+ Field type must satisfy `reflect.CanInt()`.
+ `spec` gains `noarg` (forces cardinality of 0) and `repeated` flags.
+ Short option must be provided if the field is more than one character
  long.
+ Reasonably comprehensive tests.

+ Maybe allow defaulting the short option to the first character of
  the field name if not provided?