Support {gather} and {gather*} (#2183)

* Support `gather` and `gather*`

* Add bogus screenshots

* Add correct screenshots

* Fix typo

* Update screenshots

* Improve error handling

* Scope styles inside .katex{}

* Revert console.log to console.error

src/katex.less

@@ -600,6 +600,20 @@
     .anglpad {
         padding: 0 0.03889em 0 0.03889em; // pad 1mu left and right (in scriptstyle)
     }
+
+    .eqn-num::before {
+        counter-increment: katexEqnNo;
+        content: "(" counter(katexEqnNo) ")";
+    }
+
+    .mml-eqn-num::before {
+        counter-increment: mmlEqnNo;
+        content: "(" counter(mmlEqnNo) ")";
+    }
+
+    .mtr-glue {
+        width: 50%;
+    }
 }
 
 .katex-display {
@@ -635,3 +649,9 @@
     text-align: left;
     padding-left: 2em;
 }
+
+// Automatic equation numbers for some environments.
+// Use parallel counters for HTML and MathML.
+body {
+    counter-reset: katexEqnNo mmlEqnNo;
+}