Make skip_empty variable overridable per window

The denite extension functions return the content of some buffer-local
variables. Those variables are not defined, the first time the they are
accessed and therefore, the statusline is not updated later when
g:airline_skip_empty_sections is set.

So disable this variable in this window, by setting the
w:airline_skip_empty_section=0 variable in the denite window.

closes #1454

autoload/airline/builder.vim

@@ -161,6 +161,11 @@ function! s:section_is_empty(self, content)
   if get(g:, 'airline_skip_empty_sections', 0) == 0
     return 0
   endif
+
+  " only check, if airline#skip_empty_sections == 1
+  if get(w:, 'airline_skip_empty_sections', 0) == 0
+    return 0
+  endif
   " assume accents sections to be never empty
   " (avoides, that on startup the mode message becomes empty)
   if match(a:content, '%#__accent_[^#]*#.*__restore__#') > -1

autoload/airline/extensions/denite.vim

@@ -19,6 +19,7 @@ endfunction
 
 function! airline#extensions#denite#apply(...)
   if &ft == 'denite'
+    let w:airline_skip_empty_sections = 0
     call a:1.add_section('airline_a', ' Denite %{airline#extensions#denite#check_denite_mode('.a:2['bufnr'].')}')
     call a:1.add_section('airline_c', ' %{denite#get_status_sources()}')
     call a:1.split()

doc/airline.txt

@@ -148,6 +148,12 @@ values):
  >
   let g:airline_skip_empty_sections = 1
 <
+
+  This variable can be overriden by setting a window-local variable with
+  the same name (in the correct window):
+ >
+  let w:airline_skip_empty_sections = 0
+<
 ==============================================================================
 COMMANDS                                                  *airline-commands*