version: finish version setup

This setup will now correctly set the version number from the git tag.
When using `--version`, we will see the binary name, the package it was
built from and a git hash based on the tag:

```console
$./bin/dist -v
./bin/dist github.com/docker/containerd 0b45d91.m
```

Note that in the above example, if we set a tag of `v1.0.0-dev`, that
will show up in the version number, as follows:

```console
$./bin/dist -v
./bin/dist github.com/docker/containerd v1.0.0-dev
```

Once commits are made past that tag, the version number will be
expressed relative to that tag and include a git hash:

```console
$./bin/dist -v
./bin/dist github.com/docker/containerd v1.0.0-dev-1-g7953e96.m
```

Some these examples include a `.m` postfix. This indicates that the
binary was build from a source tree with local modifications.

We can add a dev tag to start getting 1.0 version numbers for test
builds.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/ctr/main.go

@@ -9,6 +9,12 @@ import (
 	"github.com/urfave/cli"
 )
 
+func init() {
+	cli.VersionPrinter = func(c *cli.Context) {
+		fmt.Println(c.App.Name, containerd.Package, c.App.Version)
+	}
+}
+
 func main() {
 	app := cli.NewApp()
 	app.Name = "ctr"