support new levelup encoding pattern

[dominictarr]

in the next release, levelup will support custom encodings.

Level/levelup#149

this pull request means that bytewise can be dropped in to an encoding field as a custom encoding.

levelup(path, {keyEncoding: require('bytewise'), valueEncoding: 'binary'})

The important addition is buffer: true which tells levelup to retrive a binary value when doing a read to a custom encoded field.

Also, I've added bytewise-hex encoding:

levelup(path, {keyEncoding: require('bytewise/hex'), valueEncoding: 'binary'})

this is only a few extra lines, but if you need a string, then you need hex.
(base64 is not useful, because order is not consistent)

Currently, this is needed for sublevel - however, soon I will refactor sublevel to support binary keyEncodings.

[deanlandolt]

Thanks for the awesome commit, @dominictarr!