Add the '\rule' command for drawing boxes

Summary:
Supports the 'ex' and 'em' units for sizes. Doesn't support the optional depth
argument.

Test Plan:
 - See that the huxley test looks good, and nothing else changed
 - See that the tests pass

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12777

static/katex.less

@@ -305,6 +305,11 @@ big parens
         }
     }
 
+    .rule {
+        display: inline-block;
+        border-style: solid;
+    }
+
     .overline {
         .baseline-align-hack-outer;