[stex mode] Add inMathMode config option

When `true` `sTeX` parser starts in math mode.

Author: AlbertHilb

mode/stex/index.html

@@ -103,6 +103,12 @@ <h2>sTeX mode</h2>
       var editor = CodeMirror.fromTextArea(document.getElementById("code"), {});
     </script>
 
+    <p>sTeX mode supports this option:</p>
+    <d1>
+      <dt><code>inMathMode: boolean</code></dt>
+      <dd>Whether to start parsing in math mode (default: <code>false</code>).</dd>
+    </d1>
+
     <p><strong>MIME types defined:</strong> <code>text/x-stex</code>.</p>
 
     <p><strong>Parsing/Highlighting Tests:</strong> <a href="../../test/index.html#stex_*">normal</a>,  <a href="../../test/index.html#verbose,stex_*">verbose</a>.</p>

mode/stex/stex.js

@@ -16,7 +16,7 @@
 })(function(CodeMirror) {
   "use strict";
 
-  CodeMirror.defineMode("stex", function() {
+  CodeMirror.defineMode("stex", function(_config, parserConfig) {
     "use strict";
 
     function pushCommand(state, command) {
@@ -173,7 +173,7 @@
       if (source.eatSpace()) {
         return null;
       }
-      if (source.match(endModeSeq)) {
+      if (endModeSeq && source.match(endModeSeq)) {
         setState(state, normal);
         return "keyword";
       }
@@ -235,9 +235,10 @@
 
     return {
       startState: function() {
+        var f = parserConfig.inMathMode ? function(source, state){ return inMathMode(source, state); } : normal;
         return {
           cmdState: [],
-          f: normal
+          f: f
         };
       },
       copyState: function(s) {