feat(versioncmd): adds version command to return current version

[drubin]

Adds a version command so it matches all a bunch of other tools.

garden version and garden -v return the same result.

[eysi09]

Actually, setting the implicitCommand flag to true in this line allows you to call garden version with the desired outcome. However, that won't print version in the command list when running garden --help. Same applies to help. My apologies for not having noticed this sooner.

[drubin]

@eysi09 I tried that too. I would prefer not to tie us to sywac more than we have to since #393

Also I see garden -v and garden version differing slightly as things progress to client/server/api model.

docker -v
Docker version 18.09.0, build 4d60db4

vs

docker version
Client: Docker Engine - Community
 Version:           18.09.0
 API version:       1.39
 Go version:        go1.10.4
 Git commit:        4d60db4
 Built:             Wed Nov  7 00:47:43 2018
 OS/Arch:           darwin/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          18.09.0
  API version:      1.39 (minimum version 1.12)
  Go version:       go1.10.4
  Git commit:       4d60db4
  Built:            Wed Nov  7 00:55:00 2018
  OS/Arch:          linux/amd64
  Experimental:     true

[edvald]

Do we need to keep the -v flag? I tend to confuse it as a short-hand for --verbose which I got used to using some other tools.

[drubin]

I don't have a preference, although right now I would err on leaving it there. Would want to make it as easy as possible to get the version out also feels a bit premature to just remove it.