User-configurable setting for "active session update frequency"

[NudeSpoons]

Updating the currently active session every 15-30s would be great progress.

Originally posted by @edouard-lopez in https://github.com/ReDEnergy/SessionSync/issues/3#issuecomment-121608568

I agree with this suggestion, though it would be better if the user could override any built-in (default) update frequency (for the currently active session) through a configuration setting.