Customizable makeprg

It would be nice if the makeprg used by this plugin would be customizable. I often work in projects where running the file I'm currently editing is not very interesting but what's really interesting is starting the server.js file. I have made a node-makeprg program that handle that case and manually modified the `javascript.vim` file to use it. It would be nice however if this plugin would use a config variable to set the makeprg to use.

https://github.com/olalonde/node-makeprg