[Improvement] Allow definition of server.hostname on the command-line

[sebbASF]

When used with Docker, the value for server.hostname needs to be set to 0.0.0.0.
This currently requires making a copy of jbake.properties with the required change, and using the -c option to provide the updated copy. Rather awkward just to define a single property.

It would be useful if the value could be defined on the command-line.

The simplest would be to have a single boolean option which sets server.hostname=0.0.0.0. (e.g. -docker).
More flexible would be to allow the value to be provided on the command-line.
Even more flexible would be to allow arbitrary properties to be defined on the command-line.