Implements \mathchoice command (#969)

This PR implements `\mathchoice` function.
I once created PR on the wrong branch. Sorry for the mess.

This is particularly useful when one defines custom macro for "big operators".
For example:

```latex
\newcommand{\infdisj}{%
  \mathop{%
    \mathchoice{% display
      \bigvee\hspace{-2ex}\bigvee%
    }{          % inline
      \bigvee\hspace{-1.75ex}\bigvee%
    }{          % script
      \bigvee\hspace{-1.4ex}\bigvee%
    }{          % scriptscript
      \bigvee\hspace{-1ex}\bigvee%
}}}
```

test/katex-spec.js

@@ -2660,3 +2660,31 @@ describe("The maxSize setting", function() {
         expect(built.style.borderTopWidth).toEqual("0em");
     });
 });
+
+describe("The \\mathchoice function", function() {
+    const cmd = "\\sum_{k = 0}^{\\infty} x^k";
+
+    it("should render as if there is nothing other in display math", function() {
+        const plain = getBuilt("\\displaystyle" + cmd)[0];
+        const built = getBuilt(`\\displaystyle\\mathchoice{${cmd}}{T}{S}{SS}`)[0];
+        expect(built).toEqual(plain);
+    });
+
+    it("should render as if there is nothing other in text", function() {
+        const plain = getBuilt(cmd)[0];
+        const built = getBuilt(`\\mathchoice{D}{${cmd}}{S}{SS}`)[0];
+        expect(built).toEqual(plain);
+    });
+
+    it("should render as if there is nothing other in scriptstyle", function() {
+        const plain = getBuilt(`x_{${cmd}}`)[0];
+        const built = getBuilt(`x_{\\mathchoice{D}{T}{${cmd}}{SS}}`)[0];
+        expect(built).toEqual(plain);
+    });
+
+    it("should render  as if there is nothing other in scriptscriptstyle", function() {
+        const plain = getBuilt(`x_{y_{${cmd}}}`)[0];
+        const built = getBuilt(`x_{y_{\\mathchoice{D}{T}{S}{${cmd}}}}`)[0];
+        expect(built).toEqual(plain);
+    });
+});