Alignment CSS class is to ambiguous on its own

[Piralein]

Problem:

The PR #40 hard-coded the align-* classes to the container surrounding the video element.
This classes are already used in different themes or may be used as part of custom themes with different CSS definitions or use cases as the ones assumed by this extension.

It is currently not possible to add an additional class to the container with the configuration, this results in the container being not distinguishable as part of this extension.

Proposal:

Add an additional class to the container to properly mark it as part of this extension, so its CSS definitions can be overridden.
.align-left.sphinx-contrib-video-container {...}

Or add an additional :class: config for the container.

[12rambau]

I think the easiest would be to add the class parameter to the container. I aligned the align-* class to match the one used by Sphinx for images.

Would you be ok to make a PR ?