Added support for bold italic symbols (#1011)

* added support for \boldsymbol

* added \boldsymbol tests

* added \boldsymbol screenshots

* added \bm, snapshot, updated screenshots

* updated snapshot for \boldsymbol

* properly display \imath and 2 in boldsymbol

* implemented \boldsymbol for +

* fix typo, remove boldsymbolLetters hardcode

* make comment about using Main-Bold more general

* fixed trailing space CI error

test/mathml-spec.js

@@ -75,4 +75,9 @@ describe("A MathML builder", function() {
         expect(getMathML(`x_{y_{\\mathchoice{D}{T}{S}{${cmd}}}}`))
             .toMatchSnapshot();
     });
+
+    it("should render boldsymbol with the correct mathvariants", () => {
+        expect(getMathML(`\\boldsymbol{Ax2k\\omega\\Omega\\imath+}`))
+            .toMatchSnapshot();
+    });
 });