Add docs for skipAnimation, re-add skipAnimation in customAction

[nytamin]

Background:

The skipAnimation was initially added to all actions in PR #8, however I had forgotten to update the specification document in that PR.

Then (due to it missing in the specification document), skipAnimation was removed from customAction() in PR #34.

This PR

• Adds a missing definition of the skipAnimation property in updateAction(), in the specification document.
• Re-adds the previously removed skipAnimation in customAction().

Motivation:

To expand on #8, I think it would be beneficial if the skipAnimation property is present for all methods (excluding the non-realtime ones).

For example, a Graphics Controller can then easilly display a graphics preview to a user by simply including skipAnimation: true to all commands on a "preview channel".

[dvdeurse]

Yes, I omitted it from the JSON schema because I thought the text contained what we agreed. However, I agree with this PR. Initially, I doubted that we should do this for customAction() (as this could be anything, not necessarily related to rendering on screen), but the use case makes sense. If we would not have such a parameter for customAction(), the preview use case without animation would be hard to implement when there are custom actions defined.