Allow to skip sending entirely

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Allow multiple subscribers

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Allow adding subscribers

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Don't allow subscribers to duplicate

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>