(Most of these changes are aimed at making matrix not a pain to use)
- The `defer_joins` option on prpl accounts waits for users to be added
  to a prpl conversation before adding the bitlbee user, in the hope
  of being able to get some chat name/alias metadata before doing so.
- (To that end, the name hint for rooms is now updated when users are
  added.)
- The `name_with_tag` options prefixes the account tag to the name hint
  for prpl accounts, in order to namespace prpl channels somewhat.
- The `use_matrix_alias` option is specialized for the purple-matrix
  plugin, and makes the name hint use the `matrix_alias` data item
  of the prpl conversation, instead of the title.

- The new `mark_read` option controls whether messages are marked
  as read upon reception by bitlbee.
- Not all libpurple prpls actually support this, but the ones that
  do (e.g. slack-libpurple) seem to be watching out for a
  conversation update of type PURPLE_CONV_UPDATE_UNSEEN, so we
  send that on message reception if the option is enabled.
- Additionally, some (like purple-facebook) check for has_focus, so
  we always return TRUE for that if mark_read is enabled.