Make plugin reloadable

there was a call to s:init() missing in the toggle function.
Without it, it wouldn't reload correctly, since the s:initialized
variable would never be set to 1 again. So load it when setting up
everything correctly.

fixes #1842

plugin/airline.vim

@@ -88,6 +88,7 @@ function! s:airline_toggle()
 
     call airline#util#doautocmd('AirlineToggledOff')
   else
+    call s:init()
     let s:stl = &statusline
     augroup airline
       autocmd!