Add bash autocompletion generator

Given a (potentially annotated) cobra command you can generate a bash
completion script.

bash_completions_test.go

@@ -0,0 +1,74 @@
+package cobra
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+)
+
+var _ = fmt.Println
+var _ = os.Stderr
+
+func check(t *testing.T, found, expected string) {
+	if !strings.Contains(found, expected) {
+		t.Errorf("Unexpected response.\nExpecting to contain: \n %q\nGot:\n %q\n", expected, found)
+	}
+}
+
+// World worst custom function, just keep telling you to enter hello!
+const (
+	bash_completion_func = `__custom_func() {
+COMPREPLY=( "hello" )
+}
+`
+)
+
+func TestBashCompletions(t *testing.T) {
+	c := initializeWithRootCmd()
+	cmdEcho.AddCommand(cmdTimes)
+	c.AddCommand(cmdEcho, cmdPrint)
+
+	// custom completion function
+	c.BashCompletionFunction = bash_completion_func
+
+	// required flag
+	c.MarkFlagRequired("introot")
+
+	// valid nounds
+	validArgs := []string{"pods", "nodes", "services", "replicationControllers"}
+	c.ValidArgs = validArgs
+
+	// filename extentions
+	annotations := make([]string, 3)
+	annotations[0] = "json"
+	annotations[1] = "yaml"
+	annotations[2] = "yml"
+
+	annotation := make(map[string][]string)
+	annotation[BashCompFilenameExt] = annotations
+
+	var flagval string
+	c.Flags().StringVar(&flagval, "filename", "", "Enter a filename")
+	flag := c.Flags().Lookup("filename")
+	flag.Annotations = annotation
+
+	out := new(bytes.Buffer)
+	c.GenBashCompletion(out)
+	str := out.String()
+
+	check(t, str, "_cobra-test")
+	check(t, str, "_cobra-test_echo")
+	check(t, str, "_cobra-test_echo_times")
+	check(t, str, "_cobra-test_print")
+
+	// check for required flags
+	check(t, str, `must_have_one_flag+=("--introot=")`)
+	// check for custom completion function
+	check(t, str, `COMPREPLY=( "hello" )`)
+	// check for required nouns
+	check(t, str, `must_have_one_noun+=("pods")`)
+	// check for filename extention flags
+	check(t, str, `flags_completion+=("_filedir '@(json|yaml|yml)'")`)
+}