Use default configuration if there are missing keys in the config file

[njbirth]

Currently, it is required that every key is present in the user config file (usually ~/.config/cocopye/cocopye.toml). If there are missing keys (which might easily happen if new options are added in an update), CoCoPyE crashes and the user has to delete the file or manually add the missing keys, which is very inconvenient.

Possible solution: Merge the user config file with the default configuration. (It might be necessary to switch the toml parser as tomlkit doesn't seem to support merging configurations.)