Allow customizing package versions

[jmuk]

Packman bundles the version values and dependencies within its own package, and there is no way to customize it.

This makes sense for dependencies, but this also means that to update the package versions, we need to modify the configs and re-upload the package on npmjs.com. That is not quite handy.

We should allow customizing the version values in config/api_defaults.yml file through a commandline flag?

Thoughts? @bjwatson @tbetbetbe