cli: add option of skipping actions

[ivotron]

Add ability in popper run of skipping one or more actions (via a --skip flag). This will run all the workflow, except the given list of actions. If, after removing all the skipped actions, the remaining workflow contains a "gap" (i.e. an action that depends on one of the skipped actions), then popper run needs to throw an error notifying users about it.

[JayjeetAtGithub]

@ivotron Are we required to pass a , separated list of action names to the --skip flag?
Ex : popper run --skip a,b,c

[ivotron]

yeah, that would be useful. The other alternative would be to allow --skip multiple times, but I'm not sure if click supports that (I believe it does).