Add CLI commands to access snapshot functionality

[evanphx]

This adds access to the snapshot API functionality. It supports reading and writing to and from files, as well as standard input and output.

Closes #682

As we don't yet have a CLI testing framework, I've included the output from adhoc testing here for review:

~/hc-git/waypoint f-snapshot-cli*
❯ ./waypoint server snapshot
failed to open output: stdout is a terminal, refusing to pollute (use '-' to force)

~/hc-git/waypoint f-snapshot-cli* 36s
❯ ./waypoint server snapshot out1.snap
Snapshot written to 'out1.snap'

~/hc-git/waypoint f-snapshot-cli*
❯ less out1.snap
"out1.snap" may be a binary file.  See it anyway?

~/hc-git/waypoint f-snapshot-cli*
❯ ./waypoint server snapshot > out2.snap

~/hc-git/waypoint f-snapshot-cli*
❯ shasum out1.snap
cc4fd6cdc75e3da5f51f5664a310a75814052d34  out1.snap

~/hc-git/waypoint f-snapshot-cli*
❯ shasum out2.snap
cc4fd6cdc75e3da5f51f5664a310a75814052d34  out2.snap

~/hc-git/waypoint f-snapshot-cli*
❯ ./waypoint server restore out1.snap
Server data restored from 'out1.snap'.

~/hc-git/waypoint f-snapshot-cli*
❯ ./waypoint server restore
failed to open output: stdin is a terminal, refusing to use (use '-' to force)

~/hc-git/waypoint f-snapshot-cli*
❯ ./waypoint server restore < out1.snap
Server data restored.

[mitchellh]

Looks super good. Some wording comments and then suggesting supporting -exit

[briancain]

Looks good to me! 👍 I found a typo in the help text so requesting changes.

[briancain]

🎉