Function line_is_url

fn line_is_url(is_error_code: bool, columns: usize, line: &str) -> bool

Returns true if line appears to be a line comment containing a URL, possibly with a Markdown link label in front, and nothing else. The Markdown link label, if present, may not contain whitespace. Lines of this form are allowed to be overlength, because Markdown offers no way to split a line in the middle of a URL, and the lengths of URLs to external references are beyond our control.