ArgumentContext.ignore should mangle the options list for ignored arguments

[tjprescott]

If I have:

with ArgumentContext('foo') as c:
  c.argument('arg1', options_list=['--this'])

with ArgumentContext('foo bar') as c:
   c.ignore('arg1')
   c.argument('arg2', options_list=['--this'])

I will get an error because, although arg1 would be ignored, argparse sees a name conflict between arg1 and arg2. If the ignore method automatically changed the options_list to something else --__ARG1 for instance, then it won't produce the conflict and avoids me having to do this:

with ArgumentContext('foo') as c:
  c.argument('arg1', options_list=['--this'])

with ArgumentContext('foo bar') as c:
   c.argument('arg1', ignore_type, options_list=['--literally-anything-else'])
   c.argument('arg2', options_list=['--this'])