Fix issues around new configs

[alexfikl]

This (hopefully) ensures that the config is properly set up in the following situations:

• no config file exists => add some default sections and libraries in there.
• no [settings] section exist => add one and make a best guess at a default-library.
• add some warnings for these scenarios so the user isn't totally surprised.

Fixes #496.

[alejandrogallo]

Thank you Alex, I apologize as this bug has been there for a while and I did not get to go about it, this is very cleanly done and also with the tests

[alexfikl]

Thank you Alex, I apologize as this bug has been there for a while and I did not get to go about it, this is very cleanly done and also with the tests

No worries at all! Thank you for taking a look 🎉