add deprecation warnings.

autoload/airline/deprecation.vim

@@ -0,0 +1,21 @@
+" MIT License. Copyright (c) 2013 Bailey Ling.
+" vim: et ts=2 sts=2 sw=2
+
+function! airline#deprecation#check()
+  if exists('g:airline_enable_fugitive') || exists('g:airline_fugitive_prefix')
+    echom 'The g:airline_enable_fugitive and g:airline_fugitive_prefix variables are obsolete. Please read the documentation about the branch extension.'
+  endif
+
+  let tests = [
+        \ [ 'g:airline_paste_symbol', 'g:airline_symbols.paste' ],
+        \ [ 'g:airline_readonly_symbol', 'g:airline_symbols.readonly' ],
+        \ [ 'g:airline_linecolumn_prefix', 'g:airline_symbols.linenr' ],
+        \ [ 'g:airline_branch_prefix', 'g:airline_symbols.branch' ],
+        \ ]
+  for test in tests
+    if exists(test[0])
+      echom printf('The variable %s is deprecated and may not work in the future. It has been replaced with %s', test[0], test[1])
+    endif
+  endfor
+endfunction
+