[new_options] added verticalOffset and parentSelector options

While using this plugin on a project of mine, I required 2 new options.

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)

A few notes on the pull request:

- Apologies for all of the whitespace changes—my editor is set to remove trailing space and use soft tabs.
- Turned all options into an options hash (as requested a few months ago)
- Updated the `README` and `index.html` example with the new options

Feel free to shoot any questions my way. Cheers!