Add note on using itertools.dropwhile() for efficiency in _is_allowed().

* If/when it's rewritten, a simple speed/efficiency check can be done with:
   >>> import dis
   >>> dis.dis(_is_allowed_orig('--dragons'))
   >>> dis.dis(_is_allowed_orig('--encrypt'))
   >>> dis.dis(_is_allowed_new('--dragons'))
   >>> dis.dis(_is_allowed_new('--encrypt'))

src/_parsers.py

@@ -404,6 +404,7 @@ def _is_allowed(input):
                 hyphenated = _hyphenate(input)
         else:
             hyphenated = input
+            ## xxx we probably want to use itertools.dropwhile here
             try:
                 assert hyphenated in allowed
             except AssertionError as ae: