Add blankLine method to mode API

So that languages in which blank lines are significant can detect
them.

Author: marijnh

lib/codemirror.js

@@ -1633,6 +1633,7 @@ var CodeMirror = (function() {
     highlight: function(mode, state) {
       var stream = new StringStream(this.text), st = this.styles, pos = 0;
       var changed = false, curWord = st[0], prevWord;
+      if (this.text == "" && mode.blankLine) mode.blankLine(state);
       while (!stream.eol()) {
         var style = mode.token(stream, state);
         var substr = this.text.slice(stream.start, stream.pos);

manual.html

@@ -625,6 +625,12 @@ <h2 id="modeapi">Writing CodeMirror Modes</h2>
     CSS class string, or <code>null</code> for tokens that do not have
     to be styled.<p>
 
+    <p id="blankLine">By default, blank lines are simply skipped when
+    tokenizing a document. For languages that have significant blank
+    lines, you can define a <code>blankLine(state)</code> method on
+    your mode that will get called whenever a blank line is passed
+    over, so that it can update the parser state.</p>
+
     <p id="StringStream">The stream object encapsulates a line of code
     (tokens may never span lines) and our current position in that
     line. It has the following API:</p>