Add .pure-u-1-1 and `includeWholeNumbers` selector which adds .pure-u-1

This adds an additional `.pure-u-1-1` selector to fit with the standard
numerator-denominator format. A new `includeWholeNumbers` option as been
added (enabled by default) which will add the `.pure-u-1`.
Add support for any while numbers, not only `1/1`.