Arguments to mapget can be specified in a configuration file

[l-laura]

Currently, starting mapget with custom data sources requires lengthy command line invocations. CLI11, the command line argument parser used by mapget, can be used to parse config files containing the arguments as documented here.

The goal is for mapget to support reading arguments from a configuration file in addition to the command line, and to document this option. Existing integration tests triggered by CLI arguments should be extended to include one example config file.