Detect markdown line width, resolve relative URLs (#332)

~~this is semi-blocked by https://github.com/charmbracelet/glamour/pull/96, but behaviour isn't really worse than the previous behaviour (most links work, some are still broken)~~

#### testcase for link resolver
```
tea pr 332
tea checkout 332 && make install && tea pr 332
```

- [rel](./332)
- [abs](/gitea/tea/pulls/332)
- [full](https://gitea.com/gitea/tea/pulls/332)

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/332
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

modules/print/comment.go

@@ -13,20 +13,27 @@ import (
 
 // Comments renders a list of comments to stdout
 func Comments(comments []*gitea.Comment) {
+	var baseURL string
+	if len(comments) != 0 {
+		baseURL = comments[0].HTMLURL
+	}
+
 	var out = make([]string, len(comments))
 	for i, c := range comments {
 		out[i] = formatComment(c)
+		baseURL = comments[i].HTMLURL
 	}
+
 	outputMarkdown(fmt.Sprintf(
 		// this will become a heading by means of the first --- from a comment
 		"Comments\n%s",
 		strings.Join(out, "\n"),
-	))
+	), baseURL)
 }
 
 // Comment renders a comment to stdout
 func Comment(c *gitea.Comment) {
-	outputMarkdown(formatComment(c))
+	outputMarkdown(formatComment(c), c.HTMLURL)
 }
 
 func formatComment(c *gitea.Comment) string {