New `maximum` feature

This feature gives you more control over the progressbar.
You can specify a maximum value of progress, if you want the progress bar stopped at this value during auto incrementing.

Example:

    NProgress.max(0.3);
    NProgress.start();
    
    // Do something long and when it's done...
    somethingLong(function() {
        NProgress.set(0.3);
        NProgress.max(0.6);
        
        // Next long process...
        nextLongProcess(function() {
            NProgress.set(0.6);
            NProgress.max(1);
            
            // And at last...
            lastStand(function() {
                NProgress.done();
            })
        })
    })