closes#1730
- Make sure to always call the term extension on TermOpen autocommands
- consistently use the airline_term highlighting group
- hard code the airline_term highlighting group, because by the time the
function airline#themes#patch() is called the highlighting group
airline_c would no be defined, so we cannot dynamically get those
attributes
- In the documentation, mention how the g:airline_mode_map can be set
including the terminal section
