Add version variable and --version flag support

The root.go is missing the version variable needed for `--version` flag support.

**To add in `cmd/alexa/root.go`:**
```go
var version = "dev"
```

This gets set via ldflags during build (`-X main.version=$(VERSION)`) but needs the variable defined.

Also add to rootCmd:
```go
rootCmd.Version = version
```