[Feature] Dynamically update workflows

[TaoXieSZ]

Hi @s8sg ,

We are using goflow to build an online system. The main use case is to orchestrate different tasks together like workflows. For example, initially user has a workflow like

Node-A -> Node-B

But the user now want to modify and changed to

Node-A -> Node-C -> Node-B

Currently, however, after registering workflows, the DAG cannot be changed.

(Of course, we can restart the flow service to achieve the reload)

So I try to contribute in this PR #64