Add AlexaFlush directive

[heneryville]

Add an Alexa directive called FlushToProgressResponse that will kick off sending all of the speech gathered so far to the Alexa progressive response API, and clearing out the speech. This should replace Alexa using the say directive for progressive responses.

Some requirements:

• This directive should do nothing if there is no speech built up yet. It should not initiate any network requests in this situation.
• Any speech built up in the current transition should be added in BEFORE this directive runs. That is, this directive should execute with lower priority than the other say, ask, tell directives in the same transition.
• We can think about changing the name. However, it'll be used infrequently enough that a long descriptive name is probably OK.

[rmberrios]

I will close this issue due to inactivity