Before/After __all__ directives and allow for multiple tasks

[ryanrichholt]

Defining a task which runs "after everything else is done" can be accomplished now by using the regex flow directives: after-re: .*, before-re: .* This works by matching all other task names, and implicit removal of self-dependency directives in the workflow graph constructor.

The improvement would allow a keyword, maybe __all__, to be used with the standard flow directives. This would also allow multiple tasks to be declared as "pre" and "post" tasks. I'm still not sure how the execution order should work. I think it would be great if they fired in the same order that they were declared, but I will need to review the code and test to make sure the order is preserved when saving/modifying workflows.