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 the 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%{&modified?' +':''}"
  let g:airline_section_c = ''
```