Composite key feature

[puddleoasis]

Added support for a composite key. Created test cases. Updated ReadMe. Would love feedback. Thanks! #1

[felciano]

The design of using multiple --key parameters seems clean and simple. It avoids the need to define a distinct key separator, and presumably uses set semantics, i.e. the order in which you call --key doesn't matter.

@simonw this has my +1 to merge in