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
Merge pull request #303 from mcphail/issue_302

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