`touch` on cache file fails when parent folder doesn't exist

[max-baz]

This line fails when the folder ~/.config/termpdf doesn't exist.

https://github.com/dsanson/termpdf/blob/5d9e9796d618dbe81bace9ffeb65f409cb461cb4/termpdf#L295

However, ~/.config in general is not a good place for storing cache, please store it in ~/.cache/termpdf.