Add flag to switch over to using CIDv1

[whyrusleeping]

To make it the switch over easier, lets get an option in as soon as possible to make go-ipfs use and generate version 1 CIDs for all internally created data. This will still be using the existing protobuf encoding for everything, the only difference will be that instead of a bare multihash (CIDv0) being used to address blocks and dags, we will use a full CID

[whyrusleeping]

@kevina this is done, yeah?

[kevina]

@whyrusleeping Depends on your definition of done. The merged p.r., #3743, only adds CidV1 support to the add command, other commands might need it and I also think it should be a global option not one local to add.

[whyrusleeping]

@kevina fair enough, lets keep this one open.