instead of enum
Additional categories can be added, which would cause a crash when
decoding.
As the category isn't used for anything, storing it as an enum value is
not necessary.
UITextView uses TextKit internally, unlike UILabel, so no additional
code is needed to keep the TextKit and view representations of the text
in sync since they are one and the same. This means that detecting which
character was tapped in a content text view is much more accurate, which
means link handling is substantially imrpoved.
Fixes#20