Feature: Elsa 2.0 Subgraph in IWorkflow

[Rafael94]

Create a subgraph in IWorkflow so that a specific sequence of activities can be attached.

var nvRemainer = workflow
	.Add<StartAt>(...)
	.Then<TestActivity>();

nvRemainer should be StartAt if possible, instead of TestActivity.

#507

[craigfowler]

Note: This issue was created as a follow-up to a discussion which was had in #507. Most of the information about the feature request is currently in that linked issue.