add filter for extracting a JWT payload

[mjhm]

This is too special case for this library and hauls in too much jsonwebtoken machinery. But it can be easily done with an extension, which is an excellent practical example:

jwt = require 'jsonwebtoken'
_ = matchPattern.getLodashModule()

_.mixin {
  jwtDecode: (token) ->
    jwt.decode token, {json: true, complete: true}
}