Adds a config option to stop assuming module filenames end in .js
When disableAutoJsExt: true, is set you have to pass complete file
names including extension when loading modules such as foo.njs

I am using require.js server side with node.js and I need to name my
modules .njs or .model etc to separate them from static .js files.

The noext plugin does not work serverside with node.js and I feel
assuming a file ext is unwise in general. If I missed an obvious way
to accomplish this, please let me know. Someone more familiar with
r.js code then me should check and modify this patch as needed.