Add Plivo agents for SMS and voice

Huginn can already send and receive SMS and place calls through a messaging provider, but Plivo is not supported yet. Plivo is a widely used CPaaS and many teams already use it for messaging and voice, so native Plivo agents would be a useful addition.

I would like to add two agents that follow Huginn's existing agent conventions. A PlivoAgent that sends an SMS or places a call when it receives an event, and a PlivoReceiveTextAgent that receives inbound texts from Plivo and emits them as events. Both add the plivo gem and ship with specs.

I have this implemented already and am happy to open a pull request.