Add a prefix-argument to `weechat-reload-buffer'.

The function will now load
  (or current-prefix-arg weechat-initial-lines)
lines.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Extend NEWS.org

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>