Add interactive demo page to show how markdownlint handles user content.

demo/default.css

@@ -0,0 +1,45 @@
+html, body {
+  height: 100%;
+}
+body {
+  margin: 0px;
+  font-family: "Segoe UI", GillSans, Helvetica, sans-serif;
+  font-weight: lighter;
+}
+header {
+  font-size: 140%;
+}
+footer {
+  font-size: 90%;
+  text-align: right;
+}
+textarea {
+  padding: 0;
+  resize: none;
+}
+.flex-rows {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+}
+.flex-columns {
+  display: flex;
+  flex-direction: row;
+  width: 100%;
+}
+.flex-fill {
+  flex-grow: 1;
+  flex-basis: 0;
+}
+.inset {
+  box-sizing: border-box;
+  padding: 2px;
+}
+.outlined {
+  border: 1px solid black;
+  margin: 1px;
+}
+.overflow-auto-scroll {
+  overflow-x: auto;
+  overflow-y: scroll;
+}