allow nested hashes and doc fix

This Commit allows the use of nested hashes in the mapping function. It
also allows the use of log stash variables in both the key and value of
the map.

Additionally I updated the documentation so that it specifies the hash
syntax as opposed to the Array syntax. The array syntax worked because
it was silently converted to a hash.

This fixes #1323
If this is a derivative of that stackoverflow answer, then we need to respect the CC-BY-SA 3.0 license on the content (attribution to author missing, etc).

That said, I don't know if we want CC-BY-SA 3.0 code in this project which is entirely Apache 2.0 licensed.