[Suggest] Set default encoding

[hcoona]

I notice that RocksDB-Sharp uses UTF-8 as default encoding. This is better than ASCII as default. But I sill wonder whether there is a way to set the default encoding besides UTF-8?

I also suggest the Encoding should applied to Key & Value separately, both for the default encoding & per command encoding. It would be better to be able to assign an Encoding other to the default encoding for Key & Value separately.