Add formatter for whole string, instead of just lat and long

[tstibbs]

This pull request adds the ability to specify a formatter that formats the lat/long as a single string, rather than having to specify them individually.

For example, in the UK we commonly use OSGB as the coordinate system and format the strings something like SK 123 456 where SK refers to a particular bounding box, the 123 is the longitude within that box and the 456 is the lattitude. Clearly, the SK is as much as part of the longitude as the lattitude, and thus cannot be written separately.