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%
}}}
```

src/functions.js

@@ -566,3 +566,6 @@ defineFunction(["\\verb"], {
     throw new ParseError(
         "\\verb ended by end of line instead of matching delimiter");
 });
+
+// MathChoice
+import "./functions/mathchoice";

src/functions/mathchoice.js

@@ -0,0 +1,57 @@
+// @flow
+import defineFunction, {ordargument} from "../defineFunction";
+import buildCommon from "../buildCommon";
+import mathMLTree from "../mathMLTree";
+import Style from "../Style";
+import * as html from "../buildHTML";
+import * as mml from "../buildMathML";
+
+const chooseMathStyle = (group, options) => {
+    const style = options.style;
+    if (style.size === Style.DISPLAY.size) {
+        return group.value.display;
+    } else if  (style.size === Style.TEXT.size) {
+        return group.value.text;
+    } else if  (style.size === Style.SCRIPT.size) {
+        return group.value.script;
+    } else if  (style.size === Style.SCRIPTSCRIPT.size) {
+        return group.value.scriptscript;
+    }
+    return group.value.text;
+
+};
+
+defineFunction({
+    type: "mathchoice",
+    names: ["\\mathchoice"],
+    props: {
+        numArgs: 4,
+    },
+    handler: (context, args) => {
+        return {
+            type: "mathchoice",
+            display:      ordargument(args[0]),
+            text:         ordargument(args[1]),
+            script:       ordargument(args[2]),
+            scriptscript: ordargument(args[3]),
+        };
+    },
+    htmlBuilder: (group, options) => {
+        const body = chooseMathStyle(group, options);
+        const elements = html.buildExpression(
+            body,
+            options,
+            false
+        );
+        return new buildCommon.makeFragment(elements);
+    },
+    mathmlBuilder: (group, options) => {
+        const body = chooseMathStyle(group, options);
+        const elements = mml.buildExpression(
+            body,
+            options,
+            false
+        );
+        return new mathMLTree.MathNode("mrow", elements);
+    },
+});