Allow for variables in TTS

[hello-amal]

For some TTS commands, you will want variables. e.g., instead of saying "Hello, what can I do for you today," you might want to say "Hello {name}, what can I do for you today?" This can generalize even beyond names -- some people prefer certain communication styles while others don't (e.g., does the person even want the robot to say their name)? This issue focuses on enabling some amount of templating in TTS, so that the operator does not need to store several different variants of the same command and can instead load a set of variables (e.g., a person-specific profile) which then populates a template that they have saved in TTS.