markdown: new utility module

Since we've started using Markdown, why not a simple utility function
to start using it?

app/client/lib/markdown.ts

@@ -0,0 +1,11 @@
+import { sanitizeHTML } from 'app/client/ui/sanitizeHTML';
+import { BindableValue, DomElementMethod, subscribeElem } from 'grainjs';
+import { marked } from 'marked';
+
+export function markdown(markdownObs: BindableValue<string>): DomElementMethod {
+  return elem => subscribeElem(elem, markdownObs, value => setMarkdownValue(elem, value));
+}
+
+function setMarkdownValue(elem: Element, markdownValue: string): void {
+  elem.innerHTML = sanitizeHTML(marked(markdownValue));
+}