loadConfig: Allow the use of the `config` argument from `config::get`

As described in the documentation: https://rstudio.github.io/config/reference/get.html#arguments

The config argument allow to store and load different configurations in yml conf file. This feature is used in https://forgemia.inra.fr/in-wop/seinebasin2/-/blob/master/reports/01-naturalized_flows_CC_impact/02-calage.Rmd?ref_type=heads&plain=1#L8