Add repo view command

```
$ gh repo
Work with GitHub repositories.

A repository can be supplied as an argument in any of the following formats:
- by owner/repo, e.g. "cli/cli"
- by URL, e.g. "https://github.com/cli/cli"

Usage:
  gh repo [command]

Available Commands:
  view        View a repository in the browser
```

command/repo.go

@@ -0,0 +1,57 @@
+package command
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/cli/cli/internal/ghrepo"
+	"github.com/cli/cli/utils"
+	"github.com/spf13/cobra"
+)
+
+func init() {
+	RootCmd.AddCommand(repoCmd)
+	repoCmd.AddCommand(repoViewCmd)
+}
+
+var repoCmd = &cobra.Command{
+	Use:   "repo",
+	Short: "View repositories",
+	Long: `Work with GitHub repositories.
+
+A repository can be supplied as an argument in any of the following formats:
+- by owner/repo, e.g. "cli/cli"
+- by URL, e.g. "https://github.com/cli/cli"`,
+}
+
+var repoViewCmd = &cobra.Command{
+	Use:   "view [{<owner/repo> | <url>}]",
+	Short: "View a repository in the browser",
+	Long: `View a repository specified by the argument in the browser.
+
+Without an argument, the repository that belongs to the current
+branch is opened.`,
+	RunE: repoView,
+}
+
+func repoView(cmd *cobra.Command, args []string) error {
+	ctx := contextForCommand(cmd)
+	baseRepo, err := determineBaseRepo(cmd, ctx)
+	if err != nil {
+		return err
+	}
+
+	var openURL string
+	if len(args) == 0 {
+		openURL = fmt.Sprintf("https://github.com/%s", ghrepo.FullName(*baseRepo))
+	} else {
+		if strings.HasPrefix(args[0], "http") {
+			openURL = args[0]
+		} else {
+			openURL = fmt.Sprintf("https://github.com/%s", args[0])
+		}
+	}
+
+	fmt.Fprintf(cmd.ErrOrStderr(), "Opening %s in your browser.\n", openURL)
+	return utils.OpenInBrowser(openURL)
+}