plugin text commands: added _contextValid method: - _contextValid check if a method can be called in this context, according to docstring declaration - use it to parse results in /help command - use it avoid use of command in wrong context, and help message is sent in this case

# TODO: remove this when RSM and MAM are in wokkel
import wokkel
from sat.tmp.wokkel import delay as tmp_delay, pubsub as tmp_pubsub, rsm as tmp_rsm, mam as tmp_mam
wokkel.delay = tmp_delay
wokkel.pubsub = tmp_pubsub
wokkel.rsm = tmp_rsm
wokkel.mam = tmp_mam