vim-dirvish: Fixed bugs and added better include guards

Fixes issue #2142.

These changes primarily focused on making the include guards more robust
and simplify how the path for w:airline_section_c was generated.
Credits goes to @kazukazuinaina for improving on my original
implementation.

autoload/airline/extensions.vim

@@ -445,7 +445,7 @@ function! airline#extensions#load()
     endfor
   endif
 
-  if exists(':Dirvish')
+  if exists(':Dirvish') && get(g:, 'airline#extensions#dirvish#enabled', 1)
     call airline#extensions#dirvish#init(s:ext)
     call add(s:loaded_ext, 'dirvish')
   endif