Added changes to understand alignment metadata for each cells.

To fully understand this PR you need more info from https://github.com/ipython/ipython/pull/3432

Basically, this PR modified and add reveal_related jinja templates to implement the alignment of element at the cell level.

With this PR, the user can get more powerful reveal-based slideshows because now is able to organize different element with horizontal alignment.

This PR is the complement to the https://github.com/ipython/ipython/pull/3432

Cheers.

Damián.