terminal: Some improvements to Terminal detection

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

doc/airline.txt

@@ -184,6 +184,7 @@ values):
       \ 's'  : 'S',
       \ 'S'  : 'S',
       \ '' : 'S',
+      \ 't'  : 'T',
       \ }
 <
 * define the set of filename match queries which excludes a window from having