Support for dependency description

This commit updates the configuration format and JSON metadata output
to support an additional description attribute.

The description attribute is meant to further describe the purpose of the
dependency. This is a minor update to the JSON format that is fully
backward compatible.

Fixes gh-40
Fix potential NPE with unknown Spring Boot starter

Fixes gh-41
Revert "Fix type id"

This reverts commit 8f1ce31035cb98e2c31db2ae4cddb1e4c6169063.