Specify a default listening port about cabal-core

[cblgh]

thanks for making the issue @NuSkooler! i like the following way of solving this:

• choosing a default port and setting it in swarm.js
  • let's make sure to choose a port number that does not conflict with beaker or hyperdivision's projects & products (as there is a chance of overlap in users for cabal & beaker :)
• allow passing in a port option to cabal-core, which overrides the default port
• add a --port option to cabal-cli to make it easy for people to use in practice
• support port in config.yml (thanks kira!)

[hackergrrl]

This could also be a useful thing to support in cabal's config.yml