Added flite tts engine

[Holzhaus]

I've added the Flite TTS engine, which should be better suited for embedded systems like the Raspberry Pi:

Flite (festival-lite) is a small, fast run-time synthesis engine developed at CMU and primarily designed for small embedded machines and/or large servers. Flite is designed as an alternative synthesis engine to Festival for voices built using the FestVox suite of voice building tools.

Configuration example:

tts_engine: flite-tts
flite-tts:
  voice: 'slt'

[coveralls]

Coverage decreased (-0.61%) when pulling e3edf52 on Holzhaus:flite-tts-engine into dd418d6 on jasperproject:master.

[shbhrsaha]

Neat! Will test this weekend

[jugg1es]

I'm a total newb, but I followed the directions and the flite TTS engine worked fine for me with the proposed changes. Thanks!

[shbhrsaha]

Works well on the Pi! Good to merge in. Thanks for integrating.