Add DefaultValue text for flags

Allows a user to override the default value of a flag in the displayed help output.

Ex:
```
cli.IntFlag{
	Name: "foo, f",
	DefaultText: "random foo",
	Value: "bar",
}
```

Running `(app.name) -h` will now yield:

```
--foo value     (default: "random foo")
```

Fixes: #504

generate-flag-types

@@ -145,6 +145,7 @@ def _write_cli_flag_types(outfile, types):
             EnvVars []string
             Hidden bool
             Value {type}
+            DefaultText string
         """.format(**typedef))
 
         if typedef['dest']: