Start of category flag support

This adds what I think needs to be done to support categories for flags
but we will see if that works. It also forces the scripts to use python2
since they blow up under python3 which is becoming the default python on
many linux systems.

Small fix to app_test as well so it conforms to the new Flag interface.

generate-flag-types

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 """
 The flag types that ship with the cli library have many things in common, and
 so we can take advantage of the `go generate` command to create much of the
@@ -144,6 +144,7 @@ def _write_cli_flag_types(outfile, types):
             EnvVar string
             FilePath string
             Hidden bool
+            Category string
         """.format(**typedef))
 
         if typedef['value']:
@@ -170,6 +171,11 @@ def _write_cli_flag_types(outfile, types):
                 return f.Name
             }}
 
+            // GetHidden lets us know if the flag is hidden or not
+            func (f {name}Flag) GetHidden() bool {{
+                return f.Hidden
+            }}
+
             // {name} looks up the value of a local {name}Flag, returns
             // {context_default} if not found
             func (c *Context) {name}(name string) {context_type} {{