Expose targets custom data in the NotaryRepository API.

[endophage]

No description provided.

[cyli]

Should this also expose it via the CLI? #1146 allows adding custom data via the CLI, but do we want to display it via the CLI as well? Or export it to a file?

Maybe something like (for listing and lookups):

$ notary list <repo>
NAME               DIGEST                                                              SIZE (BYTES)    ROLE          CUSTOM DATA
----               ------                                                              ------------    ----          -----------
2.6                9ace551613070689a12857d62c30ef0daa9a376107ec0fff0e34786cedb3399b    528             targets       yes
2.7                9f08005dff552038f0ad2f46b8e65ff3d25641747d3912e3ea8da6785046561a    1374            targets
3.1                f4c7710b17089324055ac600ee4659ee730d14bc321f1c25d4ad6360b7e2644f    528             targets       yes
3.2                cbbda5d34003b6c916f7fcd4aaba46c2de47c229dfbd06c5849d163dcf99d35e    528             targets

$ notary inspect <repo> 2.6
latest sha256:58e1a1bb75db1b5a24a462dd5e2915277ea06438c3f105138f97eb53149673c4 528
custom data:
{
   ...
}