Allow user to set number of workers

[mbartn]

Updater class allows to set workers variable - it corresponds to Amount of threads in the thread pool for functions decorated with @run_async.
4 workers may be not optimal for a user if he does have more threads on his CPU. We could allow him to set optional variable workers from command line.
Please note that this can be useful only if someone sets up one bot instance for multiple people so this issue may not be urgent. Feel free to close if you find it not necessary