support json source maps - fixes webpack-contrib#15

Author: jantimon

index.js

@@ -14,6 +14,11 @@ module.exports = function (source, map) {
             annotation: false
         }
     };
+    
+    if (typeof map === 'string') {
+        map = JSON.parse(map);
+    }
+    
     if ( map && map.mappings ) opts.map.prev = map;
     if ( params.safe ) opts.safe = true;