Templating fix for when an escaped character is used in Javascript

If an escaped character (e.g. \n) is used in embedded Javascript inside
a template then it gets escaped to \\n. If someone has put an escaped
character in their JS then they probably want it left that way.

This commit replaces \\s with a single \ when inside evaluation tags,
reverting the previous template escaping.

underscore.js

@@ -909,7 +909,8 @@
          })
          .replace(c.evaluate || null, function(match, code) {
            return "');" + code.replace(/\\'/g, "'")
-                              .replace(/[\r\n\t]/g, ' ') + ";__p.push('";
+                              .replace(/[\r\n\t]/g, ' ')
+                              .replace(/\\\\/g, '\\') + ";__p.push('";
          })
          .replace(/\r/g, '\\r')
          .replace(/\n/g, '\\n')