Render Markdown inline code syntax in issue/pull request titles #143

There is an existing | markdown filter but the trick is limiting the rendering to just backticks for inline code, and none of the other inline syntaxes or non-inline stuff like headings or non-Markdown HTML. I did some testing and that appears to be the only Markdown syntax GitHub renders in titles: https://github.com/uncenter/github-actions-playground/issues/1 (makes sense - you don't want distracting bold, italic, strikethrough text in issue listings but inline code is practical).