Pass seneca instance on add callback

[geek]

Closes #288
Closes #194

The result is

    const seneca = Seneca({ log: 'silent' })

    seneca.add({ cmd: 'foo' }, (args, reply) => {
      reply(null, { did: reply.seneca.did })
    })

    seneca.act({ cmd: 'foo' }, function (err, result) {
      // result.did === this.did
    })

[AdrieanKhisbe]

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