Print duty hours on who is on duty? command

[alosenkov-infobip]

Use case.

When somebody asks

@cibot who is on duty?

cibot replies with the duty person name

But it's also very useful to display current duty hours, both for client transparency and own checking "what duty time do I have now?".

Reply example

<DUTY_ENGINEER_NAME> on duty today. Duty hours: from XXX to YYY (UTC)

[mr-exz]

will be available in 0.7.0