Add test for domains with underscores and fix roundtrip behavior

[kevinbackhouse]

Add a test for the underscore logic in check_domain in the autolink extension.

I had to make a minor change to the code to get the "roundtrip" test to pass. The roundtrip test checks that converting to commonmark first and then to html gives the same result as converting directly to html. The first step converts the underscore to \_ which gets handled differently by the logic in check_domain. Adding some code to skip the \ character fixes it.