* First, check the following locations in order (skip if the
corresponding environment variable is undefined or empty):
* `$ASCIINEMA_CONFIG_HOME/config`
* `$XDG_CONFIG_HOME/asciinema/config`
* `$HOME/.config/asciinema/config`
* `$HOME/.asciinema/config`
Use the first one that is found.
* If not found, create a new config file at the first location listed
above whose corresponding environment variable is nonempty; i.e.,
* Use `$ASCIINEMA_CONFIG_HOME/config` if `ASCIINEMA_CONFIG_HOME` is
nonempty;
* Next, use `$XDG_CONFIG_HOME/asciinema/config` if `XDG_CONFIG_HOME`
is nonempty;
* Finally, use `$HOME/.config/asciinema/config` if `HOME` is nonempty.
* If none of the above works, exit with error `"Need $HOME"`.
