--skip option

It would be nice to be able to skip specific tasks that, while their dependencies may technically be out of date, the results of that task should not change. This is particularly convenient when you want to rerun a very small subcomponent of an analysis.

This would have been very convenient for @bo-peng and I on more than one occasion (one of which was last night) --- how many times would we have liked to have run `workflow --skip wikitopic`?