Add initial specs/implementation for allowing translate()'s key parameter to be an array of keys ordered by priority.
Update README to show key-as-array usage.
Add options to array-key tests just to make sure the expectation is clear that all potential keys must use the same options.
Make use of exists() function that is available to us.
Merge pull request #150 from dpehrson/translate-key-array

Allow translate() to take an array of keys to test, and use the first one that exists.