make filename extension handling a bash function

We were trying to call a bash function with bash stuff like @ () from a
variable.  Stop that.  Just call a function with an arg from a variable
instead of trying to pass around the bash.

Should fix https://github.com/spf13/cobra/pull/103

bash_completions_test.go

@@ -76,7 +76,7 @@ func TestBashCompletions(t *testing.T) {
 	// 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)'")`)
+	check(t, str, `flags_completion+=("__handle_filename_extension_flag json|yaml|yml")`)
 
 	checkOmit(t, str, cmdDeprecated.Name())
 }