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/katex-spec.js

@@ -2420,6 +2420,36 @@ describe("An aligned environment", function() {
     });
 });
 
+describe("An href command", function() {
+    it("should parse its input", function() {
+        expect("\\href{http://example.com/}{example here}").toParse();
+    });
+
+    it("should allow letters [#$%&~_^] without escaping", function() {
+        const url = "http://example.org/~bar/#top?foo=$foo&bar=ba^r_boo%20baz";
+        const hash = getParsed(`\\href{${url}}{\\alpha}`)[0];
+        expect(hash.value.href).toBe(url);
+    });
+
+    it("should allow balanced braces in url", function() {
+        const url = "http://example.org/{too}";
+        const hash = getParsed(`\\href{${url}}{\\alpha}`)[0];
+        expect(hash.value.href).toBe(url);
+    });
+
+    it("should not allow unbalanced brace(s) in url", function() {
+        expect("\\href{http://example.com/{a}{bar}").toNotParse();
+        expect("\\href{http://example.com/}a}{bar}").toNotParse();
+    });
+
+    it("should allow escape for letters [#$%&~_^{}]", function() {
+        const url = "http://example.org/~bar/#top?foo=$}foo{&bar=bar^r_boo%20baz";
+        const input = url.replace(/([#$%&~_^{}])/g, '\\$1');
+        const ae = getParsed(`\\href{${input}}{\\alpha}`)[0];
+        expect(ae.value.href).toBe(url);
+    });
+});
+
 describe("A parser that does not throw on unsupported commands", function() {
     // The parser breaks on unsupported commands unless it is explicitly
     // told not to