Handle '-i' before '-g' or '-G' properly

`ag -i -g <filename_pattern>` should perform case insensitive filename search

`ag -i -G <filename_pattern> <needle>` should perform case sensitive search for needle in case insensitive filenames

`ag -G <filename_pattern> -i <needle>` should perform case insensitive search for needle in case sensitive filenames

`ag -i -G <filename_pattern> -i <needle>` should be case insensitive for both

Fix #302