Pluggable adapter/encoder api [feature idea]

Nobody has claimed this yet.

This is more a feature request / proposal... it would be nice if there was neat way to apply a transformation on incoming and outgoing data.

```js
var hyperdb = require('hyperdb')

var db = hyperdb()

// pass through
db.setEncoder(function (input) { return input })
db.setDecoder(function (input) { return input })

```
The idea is that those functions are called before any data is inserted or retrieved (before the data is returned from streams and callbacks).

There may already be a nice way to do this stuff, and the API is just a proposal at this stage. All comments and thoughts welcome!

Start by reviewing hyperdb's existing data insertion and retrieval paths, including streams and callbacks, to determine whether transformation hooks already exist. Compare those paths with the proposed setEncoder and setDecoder API; done would mean an agreed design for applying transformations consistently before insertion and before returned data.

Written by the indexing model from the issue text.