Add themes support

Usage:

  const editor = new RibbitEditor({
    themes: [
        { name: 'dark', features: { sourceMode: false } },
        { name: 'minimal', tags: minimalTags },
    ],
    currentTheme: 'dark',
  });

The built-in theme is 'ribbit-default' and is always available.
Additional themes from the themes array are registered on top.

src/static/ribbit-core.css

@@ -0,0 +1,22 @@
+/*
+ * ribbit-core.css — functional editor styles. Always load this.
+ * These styles control editor state visibility and behavior.
+ * They should not be overridden by themes.
+ */
+
+#ribbit {
+    display: none;
+}
+
+#ribbit.loaded {
+    display: block;
+}
+
+#ribbit.edit {
+    font-family: monospace;
+    white-space: pre;
+}
+
+#ribbit.wysiwyg .md {
+    opacity: 0.5;
+}