markdown: dont emit ansi sequences when not emitting to tty (#491)

Allows generating a plain text version of an issue (i.e. without colors and other terminal formatting) when storing stdout in a file.
```
tea issue --comments 1 > issue1.txt
```

`IsInteractive()` had to be moved to avoid a recursive import chain.

---

In the future, it would be nice to also respect the `--output` flag. This flag is currently designed for tabular output, but we could add more supported values like `markdown` `ansi`, `plain` to it

Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/491
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: delvh <dev.lh@web.de>

modules/print/markdown.go

@@ -16,8 +16,15 @@ import (
 // If the input could not be parsed, it is printed unformatted, the error
 // is returned anyway.
 func outputMarkdown(markdown string, baseURL string) error {
+	var styleOption glamour.TermRendererOption
+	if IsInteractive() {
+		styleOption = glamour.WithAutoStyle()
+	} else {
+		styleOption = glamour.WithStandardStyle("notty")
+	}
+
 	renderer, err := glamour.NewTermRenderer(
-		glamour.WithAutoStyle(),
+		styleOption,
 		glamour.WithBaseURL(baseURL),
 		glamour.WithPreservedNewLines(),
 		glamour.WithWordWrap(getWordWrap()),