[velocity mode] Fix parsing of variables in nested quotes

Closes codemirror#3876

Author: Steve O'Hara

mode/velocity/index.html

@@ -77,6 +77,8 @@ <h2>Velocity mode</h2>
 
 $someObject("This plus $something in the middle").method(7567).property
 
+#set($something = "Parseable string with '$quotes'!")
+
 #macro( tablerows $color $somelist )
     #foreach( $something in $somelist )
         <tr><td bgcolor=$color>$something</td></tr>

mode/velocity/velocity.js

@@ -34,7 +34,7 @@ CodeMirror.defineMode("velocity", function() {
         state.beforeParams = false;
         var ch = stream.next();
         // start of unparsed string?
-        if ((ch == "'") && state.inParams) {
+        if ((ch == "'") && !state.inString && state.inParams) {
             state.lastTokenWasBuiltin = false;
             return chain(stream, state, tokenString(ch));
         }