Add support for -version=raw to print the Go version.Info

This can be helpful to print the internal fields of the version
information that will contain more details about the git tree than the
short -version output will.

Tested:
- Tested getting the raw version:
  $ _output/go/bin/kubecfg -version=raw
  version.Info{Major:"0", Minor:"1+", GitVersion:"v2.2.1-33-gdc4becd9765393", GitCommit:"dc4becd9765393fa05a3eb06f6af9e00068fe0c5", GitTreeState:"dirty"}

- Tested getting the simple version:
  $ _output/go/bin/kubecfg -version
  Kubernetes version 0.1+, build dc4becd9765393fa05a3eb06f6af9e00068fe0c5

- Tested getting the help output:
  $ _output/go/bin/kubecfg 2>&1 | grep -- -version
    -version=false: Print version information and quit

- Made sure -version=true and -version=false works, that -version with
  an invalid argument works as expected (prints usage help) and that
  -version followed by space will not try to use the next word as its
  argument (i.e. `-version raw` prints the version but not the raw one.)

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Rename `pkg/version/flag` to `pkg/version/verflag`

This avoids some conflict with the built-in `flag` module in Go. The
module was already being renamed to `verflag` on import anyways, so we
might as well just call it that.

Tested:
- hack/build-go.sh and ran the resulting binaries with -version args.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #1121 from filbranden/rawversion2

Add support for -version=raw