[WIP] Adding interval feature to timer

[wamsachel]

Overview

• Would like to ability to set a timer to beep at every time interval (e.g. set a timer for every 60 seconds)
• For now the timer will run indefinitely until mycroft is told to stop the timer

[krisgesling]

Hey there, this is a great idea.

We've had similar requests in the past, so thanks for tackling it :)