PR listing: add --fields & expose additional fields (#415)

This PR adds the `--fields` flag to `tea pr ls` (#342), and exposes more fields specific to the `PullRequest` type:
```
   --fields value, -f value   Comma-separated list of fields to print.
                              Available values:
                              index,state,author,author-id,url,title,body,mergeable,base,base-commit,head,diff,patch,created,updated,deadline,assignees,milestone,labels,comments
                              (default: "index,title,state,author,milestone,updated,labels")

```

Co-authored-by: justusbunsi <61625851+justusbunsi@users.noreply.github.com>
Co-authored-by: Norwin <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/415
Reviewed-by: Norwin <noerw@noreply.gitea.io>
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-committed-by: justusbunsi <justusbunsi@noreply.gitea.io>

modules/print/formatters.go

@@ -72,3 +72,16 @@ func formatUserName(u *gitea.User) string {
 	}
 	return u.FullName
 }
+
+func formatBoolean(b bool, allowIcons bool) string {
+	if !allowIcons {
+		return fmt.Sprintf("%v", b)
+	}
+
+	styled := "✔"
+	if !b {
+		styled = "✖"
+	}
+
+	return styled
+}