feature: publishing header support

[dch]

I'm a long time user of rabbiteer but it doesn't support TLS.
It would be great if amqpcat could lift 2 features that are really useful for publishing:

$ rabbiteer -U $AMQP_URI publish \
  --rpc --rpctimeout 3000 \
  --routing-key some.key \
  --content-type application/json \
  -file /tmp/stuff.json

headers

• adding a content-type header (or possibly other arbitrary headers)
• auto-detcting content-type from file extension is nice but not essential