Add `weechat-password-callback' and use it in `weechat-connect'

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
require the secret in `weechat-password-auth-source-callback'

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Merge branch 'master' into stored-password

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