support dollar sign in inline dialects

In this way it is possible to use the dollar ($) sign as a startpoint for an inline dialect.

vendor/assets/javascripts/better_markdown.js

@@ -335,7 +335,7 @@
       // __foo__ is reserved and not a pattern
       if ( i.match( /^__.*__$/) )
         continue;
-      var l = i.replace( /([\\.*+?|()\[\]{}])/g, "\\$1" )
+      var l = i.replace( /([\\.*+?|()\[\]${}])/g, "\\$1" )
                .replace( /\n/, "\\n" );
       patterns.push( i.length === 1 ? l : "(?:" + l + ")" );
     }