Expose DraftEditorBlock
Expose DraftEditorBlock

Summary:This is needed for people who are wrapping `DraftEditorBlock` in custom components.

cc #124
Closes https://github.com/facebook/draft-js/pull/173

Differential Revision: D3031572

fb-gh-sync-id: 6818f49a59ddc56edc9fa44477f706242ec9ae06
shipit-source-id: 6818f49a59ddc56edc9fa44477f706242ec9ae06

SciMts/draft-js,johnmpotter/draft-js,TripleDip/draft-js,davidchang/draft-js,TripleDip/draft-js,SciMts/draft-js,dirkholsopple/draft-js,tonygentilcore/draft-js,davidchang/draft-js,johnmpotter/draft-js,tonygentilcore/draft-js,dirkholsopple/draft-js,facebook/draft-js,brookslyrette/draft-js,brookslyrette/draft-js