manage-data need to clear [name, value] pairs

[tolitius]

discovered by @MikeFair:

I went to delete a piece of managed_data using bb8 the other day; I tried { ... "fieldName": null} but bb8 didn't want to take it; is that a supported feature atm?

[tolitius]

Remove Data

Providing just the name (a.k.a. key) without the value (a.k.a. data) would remove this "name, value" pair from the account:

$ bb manage-data -s '{"source_account": "'$(cat seed)'",
                      "name": "answer to the ultimate question"}'

JSON null is also supported (would remove this "name,value" pair):

$ bb manage-data -s '{"source_account": "'$(cat seed)'",
                      "name": "answer to the ultimate question",
                      "value": null}'

committed. will release a bit later after adding #5

[tolitius]

released in 0.1.16