support descriptive entries in mcconf modules

[kubanrob]

This is not a top priority, but I think adding some descriptive entries to the modules could potentially increase the usability of McConf quite a bit.

• description: general description, useful also for searching modules. This would be useful even when not processed by McConf, as it could provide some context when examining some random module.
• tags: similar to description, but only keywords. This could be used to mark related packages (e.g. same project, userspace, kernel, ...) that have no direct dependencies.
• warning: warning to be displayed prominently when this module is included. This is for marking expensive (compile time, space), broken, deprecated or work-in-progress modules.
• error: Lets the configuration fail with an error code and display all errors encountered. This could be used for giving explicit error messages for files that should not be included. Not sure if this is a good usecase.

Feedback appreciated. I think description and warning would be immediately useful, I am not sure about tags and error.