[Notebook/Presentation] Add new special metadata tags

[ebertmi]

We could support following special metadata keys:

• spectacle.bgImage: Set a background image for the cell
• spectacle.bgDarken: Float value from 0.0 to 1.0 specifying how much to darken the bgImage image
• spectacle.bgColor: Set backgroundColor value
• align: Accepts a space delimited value for positioning interior content. The first value can be flex-start (left), center (middle), or flex-end (bottom). The second value can be flex-start (top) , center (middle), or flex-end (bottom). You would provide this prop like align="center center", which is its default.

All those attributes should be interpreted in the presentation and in the notebook. On the notebook the settings should be only interpreted for slides