Support \tag, \tag*, and \gdef (#1309)

* Tag sketch

* Drop objectAssign; already using Object.assign elsewhere

* Basic \gdef support

* Bug fix

* Finish \tag

* MathML numbers equations with <mlabeledtr>

* Fix flow bugs

* \gdef tests

* Add basic \tag tests

* Screenshot test for \tag

* \tag* test

* Add missing file

* Bug fix screenshot

* Major refactor

* Represent tag at top level of parse tree, requiring less hackery
  * No more \@tag function; it was essentially just doing \text
* Wrap tag in group so e.g. ( and ) are formatted the same
* Add `feed` method to MacroExpander for multiple inputs (for tag)
* Bug fixes in buildHTML, makeTextRow, _getBuilt (for display mode)
* Remove excess <mrow> wrapper when unnecessary

* Update screenshot from tag being wrapped in group

* Add maxExpand limit

package.json

@@ -40,7 +40,6 @@
     "less": "~2.7.1",
     "less-loader": "^4.0.5",
     "mkdirp": "^0.5.1",
-    "object-assign": "^4.1.0",
     "pako": "1.0.4",
     "pre-commit": "^1.2.2",
     "query-string": "^5.1.0",