support underlining in markdown

It became possible after https://github.com/mity/md4c/commit/e6661f23dc3eb1373e1af314ea3d6057e4a78caa

QTextMarkdownWriter::writeBlock() writes underlines if you set underlining on the char format; but QTextMarkdownImporter detects italics, because word is an alternate way of expressing italics in CommonMark. Being able to differentiate that from *emphasis* is an extension, but we can also add a QTextMarkdownImporter::Feature flag for that.