v0.5.1

RabbitMQ credential-supply fixes.

Added

• ctx set --backend rabbitmq --username to set the RabbitMQ user with the
  discrete --host/--port/--vhost flags, instead of being forced to embed it
  in --amqp-url.

Fixed

• MQGOV_PASSWORD is now honored at connection time when a context has no stored
  credential (current context and --context overrides), giving a consistent
  non-interactive password path. RabbitMQ AMQP and management API now use one
  unified username/password: explicit --username/password (or MQGOV_PASSWORD)
  overrides --amqp-url userinfo, with guest/guest as the default.