Request for assistance: mapping charsets

Matter at hand: `gh-ost`'s support for multiple charsets.

`gh-ost` works well out-of-the-box with `utf8` and `utf8mb4`.

However for other character sets it needs to be told, explicitly, how to convert strings. For example, the `latin1` character set is supported, and is converted via golang's `charmap.Windows1252`, see: https://github.com/github/gh-ost/blob/master/go/sql/encoding.go#L20

If anyone is willing to assist in mapping other MySQL charsets to golang Encoders, we're very grateful. Please PR the https://github.com/github/gh-ost/blob/master/go/sql/encoding.go file with additional charsets.