Implement sliceSortKey to sort lists of maps

Bump to v0.7.0.
Add proper godoc comments for funcs.go
Remove extraneous line in JSON loader function

No need to assign the decoder when it's only going to be used once; I think
this looks better.
Add contributing.md
Remove some extraneous variable names in GenericSlice.Less
Separate extension detection from format flag checking

Should make the flow of logic clearer in the format selection code.
Add error detection if multiple formats selected

Specifying multiple formats at once will now raise an error. I'm trying to
think of a better way to test for this, but haven't got any ideas yet.

This bugfix causes version bump to v0.7.1.