readonly mark is now a member of extensions

The read-only mark was hard-corded to be added to airline_c.
It was not configurable.

This PR will enable the users to use the readonly mark at any place they wish.
Example configuration:
```vim
  let g:airline_enable_branch = 0
  let g:airline_detect_readonly = 0
  let g:airline_section_b = "%{airline#extensions#readonly#get_mark()}%t %M"
  let g:airline_section_c = ''
```