support multiple --encoding-from and --encoding-to

[roycewilliams]

As a way to amplify work quickly, and to cover cases where source and/or target encodings are mixed, it would be very useful to specify multiple encodings.

Example usage:

--encoding-from utf8,cp1251 --encoding-to utf8,cp1251

This would produce four the equivalent of these four conversions:

--encoding-from utf8   --encoding-to utf8
--encoding-from utf8   --encoding-to cp1251
--encoding-from cp1251 --encoding-to utf8
--encoding-from cp1251 --encoding-to cp1251

As an optimization step, if the from and to are identical, the conversion might be automatically skipped.