Support for MinimumLevel.Overide()

[skomis-mm]

Implementation for #10

Proposed schema:

"MinimumLevel": {
    "Default": "{DefaultMinimumLevel}",
    "Override": {
       "{Source1}": "{MinimumLevel}",
       "{SourceN}": "{MinimumLevel}",
    }
}

or just

"MinimumLevel": "{DefaultMinimumLevel}"

[nblumhardt]

Thanks for updating this! Let's run with it - after merging this will be published in a 2.1.0-dev-nnnnn pre-release package, keen to get some feedback but I think it's a natural implementation 👍

[gzpbx]

Very useful !!