Add EndOfBuffer hl group for ~ lines after the last line in buffers

This makes it possible to highlight the lines starting with ~ at the end
of buffers and other elements highlighted using NonText.

As proposed by mhinz at
https://groups.google.com/forum/#!topic/vim_dev/p3de1iU1GXI/discussion

src/nvim/screen.c

@@ -1651,7 +1651,7 @@ static void win_update(win_T *wp)
 
     /* make sure the rest of the screen is blank */
     /* put '~'s on rows that aren't part of the file. */
-    win_draw_end(wp, '~', ' ', row, wp->w_height, HLF_AT);
+    win_draw_end(wp, '~', ' ', row, wp->w_height, HLF_EOB);
   }
 
   /* Reset the type of redrawing required, the window has been updated. */