Added output_wrapper file support.

The `--output_wrapper` option can now accept a file path to an output wrapper.
````
// this is my output wrapper (my_wrapper.js)
(function(){
%output%
})();
````
If `my_wrapper.js` is in the working directory you would simply use `--output_wrapper my_wrapper.js` and the contents of the file will be used as the output wrapper.