Improvement: Subtasks

Blather should be able to use certain key phrases after a parent phrase has been spoken. If there was a parent phrase of "Microwave" and a subtask of "reheat" then the subtask should only become available after speaking the parent phrase.

After a certain period of time the system can return to the top level so that all commands are available again.

Examples:

Refrigerator
- water
- ice
- light

Microwave
- reheat
- one minute
- two minutes
- popcorn
- light

The only time water or reheat should be available is if the user spoke the words "Refrigerator" or "Microwave" first, otherwise a smart refrigerator could be dispense water all over the floor when it misinterpreted room noise. The light action is shared between both examples but only the one within the active parent would work for a set period of time (30 seconds for example).

A key phrase could be used to return the system back to the parent level ("return to home").