Make auto-grid min-width configurable in config

[nelhop]

Instead of overriding --o-auto-grid--min-width it would be nice to define a default value in the settings.

https://github.com/lnolte/und-css/blob/587b608fd6bfe6df038621b331510290037302ca/src/objects/_auto-grid.object.scss#L3

Also it might make sense to set the default to something like var(--typography-measure--narrow), but the possibility to set the value in the settings would make this obsolete.