add dtmf-term-timeout

when using Asterisk UniMRCP.  See [Rayo](https://github.com/rayo/xmpp/pull/96) for corresponding protocol change.
We should have a test here to check that `-1` acts as _no value_ here.

That reminds me to ask @benlangfeld...  Just curious what the reason is that we have a convention in `MRCPRecogPrompt` of using `-1` as a sentinel value instead of using Nil?  I realize that switching would break the API, but wondering if this is something that we could change in the future - or if there's good reason to keep it as-is.  Thanks.