allow use of data-attributes for configuration

This allows configuration to be done with a data-attribute, instead of
using the class attribute in a nonstandard way.
Brush for Swift

This is a brush for Apple’s new Swift language. It includes matching
for keywords, \@attributes, built-in Swift-native types, and matches
string interpolations and nested multi-line comments.
Theme for Swift

This theme emulates the Apple Swift documentation syntax highlighting.