v3.0.0

This release adds the "{key:format}" syntax to FormatWith.

This is a breaking change to the current format handler API.
The format handler action that is passed to the FormatWith methods now
receives (key, format) parameters instead of just (key).

Also, if no handler is provided, the object will be formatted using the
given format string using the built in string formatters (i.e. the same
behaviour that string.Format uses).

Special thanks to https://github.com/Jmaharman for implementing this
feature.