core/commands: Make IpnsCmd and PublishCmd public

ipfs-shell [1] accesses the Command objects directly to construct requests for an external IPFS daemon API. This isn't a terribly robust approach, because it doesn't handle version differences between the version of go-ipfs used to build the daemon and the version used to build the ipfs-shell-consuming application. But for cases where you can get those APIs to match it works well. Making these two commands public allows us to write ipfs-shell wrappers for them. Until we figure out how to get ipfs-shell working without access to core/commands, I think the best approach is to make future command objects and their returned structures public, and to go back and expose existing commands/structures on an as-needed basis. In this case, I need the public PublishCmd for the Docker-registry storage driver, and I made the IpnsCmd public at the same time to stay consistent for both 'ipfs name ...' sub-commands. [1]: https://github.com/whyrusleeping/ipfs-shell License: MIT Signed-off-by: W. Trevor King <wking@tremily.us>