Generate configuration loading in Gruntfile.js

[grayside]

Currently GDT loads Gruntconfig.json as part of its internal bootstrap. This means GDT itself needs to take awkward steps to support dynamic configuration.

If GDT drops that config loading, the Gruntfile.js in the project can take over all steps to load the right configuration, both "static" from a file and dynamic from code.

GDT should still offer environment variables, to ensure consistent naming. In fact, that support should most likely be broadened if possible.

In the course of this change, we should take the opportunity to consider switching to Yaml.

This requires a companion change in GDT to remove Gruntconfig.json load and possibly rethink the "magic config" behavior.