Add update subcommand

[loongy]

It is no longer necessary to have Darknodes automatically update themselves. It is now preferable to allow Darknode operators to explicitly decide that they want to accept an update. To this end, the automatic update services have been stripped from the Darknode and we need to add an update subcommand to the CLI.

It should look like:

$ darknode update -version x.y.z

It should do the following:

• Update to latest version when there is no -version option.
• Update Darknodes by name.
• Update Darknodes by tag.
• Cannot update to version lower than the current Darknode version.
• SSH into Darknode and download binary from GitHub Release, install, and restart service.