GitLab

Try it out:

```
ipfs net diag --vis=d3 | diagnostics/d3/d3view
```

Notes: this is not the best way to do it, because it
breaks `--encoding=json`. Not sure what the best way is, and
right now this provides more utility than the other.

core/commands: Don't use pointers for Type field

core/commands: Refactored command marshalers

Also:
- map[cmds.EncodingType]cmds.Marshaller -> MarshalMap

cc @mappum @maybebtc

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

@jbenet @whyrusleeping

Docs read net-diag. It seems the command was previously registered as
diag. Which do you prefer?

because this method doesn't check actual network status

these weren't actually modified when commands2 was introduced. they're
exact replicas of the ones that presently exist in core/commands.