Add support for \text{rm,it,bf,tt,sf,normal}.

And allow \text to nest inside \text.

static/katex.less

@@ -49,6 +49,14 @@
         display: inline-block;
     }
 
+    .mathrm {
+        font-style: normal;
+    }
+
+    .textit {
+        font-style: italic;
+    }
+
     .mathit {
         font-family: KaTeX_Math;
         font-style: italic;