update readme
consider 'g:ctrlp_show_hidden'

change: update comments
extract functions
        add: s:remove_special_buffer()
        add: s:to_bufname_list()
        add: s:remove_ignored_bufname()
        add: s:get_buffers()
remove: unused variable
fix: s:remove_ignored_bufname() for g:ctrlp_show_hidden