Display which tag is being filtered in the title

go/blog/blog.go

	fmt.Fprintf(out, `
	<script>
	var baseTitle = document.title;
	var currentFilter = null;
	window.addEventListener("DOMContentLoaded", function(ev) {
				article.classList.add("does-not-match");
			}
		}
		document.title = baseTitle + " (Posts tagged '" + tag + "')";
	}
	function clearFilter() {
		}
		currentFilter = null;
		document.title = baseTitle;
	}
	</script>`)
	fmt.Fprintf(out, "\n</body>\n</html>\n")