custom encode/decode support

[jlarsson]

This PR adds support for custom encoding/decoding besides the default base64(json()) schema.

A basic test case is included.

app.use(session({
  encode: function (session) { ... },
  decode: function (encodedSession) { ... },
})

My motivation behind this is to be able to plugin custom encryption/obfuscation of the cookies.

Thanks for a great module 👍

Cheers

[dead-horse]

landed 63b46d5

🍻