Add support for meshviewer.json as well as an option for domain filter

[Kilobyte22]

I have added support for the newer meshviewer format. The format also supports multidomain, so that multiple communities share a map. To use the meshviewer format, specify the -m/--meshviewer CLI option instead of -n/--nodelist.
In such setups you will need a separate api-file per domain, so i have added an option -D/--domain to filter by this value.

This is a non-breaking change. All existing functionality remains untouched.

[spookey]

Sorry for the long delay.. I am somewhat confused anyone is still using this software I wrote years ago..

I do not have the time and resources any time soon to take care of this, so I think the best would be if you fork this repository and continue from there.
Maybe I could also transfer ownership to you completely?
But then we need to think how we migrate the CI/CD as well. First step would be to make the tests work again for your merge request: https://travis-ci.org/github/spookey/ffflash/builds/755178414