Implement a user configuration

[psss]

Seems we'll soon need a way how to store user configuration data. For example tmt test export --nitrate might need a url to the test case management system.

Brainstorming first ideas here: What about storing data under $HOME/.config/tmt in the form of an fmf tree? For the above-mentioned example we could have something like this:

/test/convert/nitrate:
    url: https://nitrate.example.com/

This would work naturaly / consistently with how the rest of data is handled in tmt.