Add mhchem extension (#1436)

* A proposal for support of mhchem

* Create extension

* Add index

* Remove arrow function

* Fix lint errors in macros.js

* Remove mhchem from build

* Move code from macros.js to mhchem.js

* Add minified mhchem

* Remove arrow head mods. Disable eslint.

* Fix space handling

* Update code to mhchem v3.3.0

* Tweak arrows to reduce diff

* Raise \tripledash

* Edit docs and webpack

* Update package.json

* Remove wrap. Remove manual.

* Correct second brush stroke in \xrightleftarrows

* Added patch file

* Add import line

* Add mhchem to eslint ignore

package.json

@@ -109,7 +109,8 @@
     "collectCoverageFrom": [
       "src/**/*.js",
       "contrib/**/*.js",
-      "!src/unicodeMake.js"
+      "!src/unicodeMake.js",
+      "!contrib/mhchem/**"
     ],
     "setupTestFrameworkScriptFile": "<rootDir>/test/setup.js",
     "snapshotSerializers": [