Added verticalOffset and parentSelector options to your jQuery-Flex-Vertical-Center plugin

While using your plugin on a project of mine, I required 2 new options, so I added them and thought you might want to include them in the plugin.

1. `verticalOffset` - the element I was centering was in a container with a floating navbar, so I had to add a vertical offset to center the element in the space **not** covered by the navbar (default: 0)

2. `parentSelector` - the element needs to be centered within a higher parent container (not the immediate parent), so I added a selector option that will find the specified selector and use the height of that element (default: immediate parent)