Allow the setting of the collation mode in the configuration file

[Xophmeister]

This discussion makes the point that it may be beneficial to be able to set the collation mode in the configuration file in certain contexts. For example, a project-specific configuration may want to override all configuration for the sake of reproducibility.

(Note: This issue depends on #583)

[ErinvanderVeen]

How would one collate the collation mode?

[Xophmeister]

As it's just a string, I would simply take its value from the highest priority config source (as with other strings), unless overridden by the CLI flag/envvar.