#7 add an "idle_callback"

[Rodrigo V. G.]

It would be useful if the library contained one more callback: an "idle_callback" which, if set, would be called at the end of every loop in the 'while' of 'irc_run(...)'.
Would need at least the session as argument.
This would be useful, among others, to defer execution of commands or other code to the future, at arbitrary times, not necessarily when some data is received from the server which triggers a regular callback.