Improve filtering by tags

We now filter when a url is entered in the address bar, and when the
browser navigation is used.  In addition, clicking on the currently
selected tag resets the filter.

go/blog/blog.go

	fmt.Fprintf(out, `
	<script>
	var currentFilter = null;
	window.addEventListener("hashchange", function(ev) {
		filterFromURL(new URL(ev.newURL));
	});
	window.addEventListener("click", function(ev) {
		if (ev.target.classList.contains("tag-link")) {
			if (ev.target.href == "") {
				return;
			}
			var u = new URL(ev.target.href);
			if (!u.hash.startsWith("#tag:")) {
				return;
			var tag = tagFromURL(new URL(ev.target.href));
			if (currentFilter == tag) {
				clearFilter();
			} else {
				filterTag(tag);
			}
			var tag = u.hash.substr(5);
		}
	});
	function filterFromURL(u) {
		var tag = tagFromURL(u);
		if (tag == null) {
			clearFilter();
		} else {
			filterTag(tag);
		}
	})
	}
	function tagFromURL(u) {
		if (!u.hash.startsWith("#tag:")) {
			return null;
		}
		return u.hash.substr(5);
	}
	function filterTag(tag) {
		currentFilter = tag;
		var articles = document.querySelectorAll("article");
		for (var i = 0; i < articles.length; i++) {
			var article = articles[i];
			}
		}
	}
	function clearFilter() {
		var articles = document.querySelectorAll("article.does-not-match");
		for (var i = 0; i < articles.length; i++) {
			articles[i].classList.remove("does-not-match");
		}
		currentFilter = null;
	}
	</script>`)
	fmt.Fprintf(out, "\n</body>\n</html>\n")
	out.Close()