Move shared package variables to cmd helper file

The Root command is just a command (though admittedly a somewhat special one). The
cmd.go file is where we're putting shared stuff.

cmd/cmd.go

@@ -3,10 +3,27 @@ package cmd
 import (
 	"fmt"
 
+	"io"
+
 	"github.com/exercism/cli/config"
 	"github.com/spf13/viper"
 )
 
+var (
+	// BinaryName is the name of the app.
+	// By default this is exercism, but people
+	// are free to name this however they want.
+	// The usage examples and help strings should reflect
+	// the actual name of the binary.
+	BinaryName string
+	// Out is used to write to information.
+	Out io.Writer
+	// Err is used to write errors.
+	Err io.Writer
+	// In is used to provide mocked test input (i.e. for prompts).
+	In io.Reader
+)
+
 const msgWelcomePleaseConfigure = `
 
     Welcome to Exercism!

cmd/root.go

@@ -2,7 +2,6 @@ package cmd
 
 import (
 	"fmt"
-	"io"
 	"os"
 	"runtime"
 
@@ -13,21 +12,6 @@ import (
 	"github.com/spf13/cobra"
 )
 
-var (
-	// BinaryName is the name of the app.
-	// By default this is exercism, but people
-	// are free to name this however they want.
-	// The usage examples and help strings should reflect
-	// the actual name of the binary.
-	BinaryName string
-	// Out is used to write to information.
-	Out io.Writer
-	// Err is used to write errors.
-	Err io.Writer
-	// In is used to provide mocked test input (i.e. for prompts).
-	In io.Reader
-)
-
 // RootCmd represents the base command when called without any subcommands.
 var RootCmd = &cobra.Command{
 	Use:   BinaryName,