Implemented `\href' command (#923)

* Implements `\href' command.

* Added `functions/href.js`.
* Added `domTree.anchor` and `buildCommon.makeAnchor` functions.
* Make `buildHTML.getTypeOfDomTree` exported.

* Reflects the code reviews

* Create new argType "url" to treat link string in math appropriately
* Stop using too shorten variable names
* Start using template strings

* Adopts template literal

* Elaborates on glueing

* If-clause restructuring

* Solved confusing explanation

* Allow balanced braces in url

* Adds unit-test for \href

* Adds snapshot tests

test/__snapshots__/mathml-spec.js.snap

@@ -220,6 +220,25 @@ exports[`A MathML builder should render mathchoice as if there was nothing 4`] =
 
 `;
 
+exports[`A MathML builder should set href attribute for href appropriately 1`] = `
+
+<math>
+  <semantics>
+    <mrow>
+      <mrow href="http://example.org">
+        <mi>
+          α
+        </mi>
+      </mrow>
+    </mrow>
+    <annotation encoding="application/x-tex">
+      \\href{http://example.org}{\\alpha}
+    </annotation>
+  </semantics>
+</math>
+
+`;
+
 exports[`A MathML builder should use <menclose> for colorbox 1`] = `
 
 <math>