sssom parse should allow `-m` to be a URL

[matentzn]

Would be great if we could get this to work:

sssom parse https://raw.githubusercontent.com/mapping-commons/sssom/master/examples/external/example1.sssom.tsv -m https://raw.githubusercontent.com/mapping-commons/sssom/master/examples/external/example1.sssom.yml -o ex.sssom.tsv

The first part already works, but the yml file at this time needs to be a local file. Would be great if it could be a URL as well.