Add a plugin system

[jhermsmeier]

So one can, for example, validate signatures (DKIM, PKCS, ...) Concept:

var Envelope = require( 'envelope' )
var DKIM = require( 'envelope-dkim' )

Envelope.use( DKIM({
  algorithm: 'rsa-sha256'
}))

var email = new Envelope( data )
email.dkim.sign()