Added a new functionality to see the arglist in the tabline

vim-airline plugin already provided the possibility to populate the
tabline with the buffer list. However the buffer list can quickly become
enormous when browsing big projects. The arglist is a more stable set of
buffers which is manually edited.

The addition to the plugin enables to display the arglist in the
tabline.
The current active arg is visually indicated in a different color (same
mechanism as vim-airline bufferlist).
This new addition defines two new functions:
    - <Plug>AirlineAddArg, which adds the current buffer to the arglist,
      thus adding a new 'tab' in the tabline.
    - <Plug>AirlineDeleteArg, which deletes the current active arg.