Support SSML in Speech-to-text

[yteraoka]

Hi,

go-chromecast is a very handy tool that I use every day on my home network.
I've tried making it even more convenient by supporting SSML.

Added --ssml flag to treat messages as SSML

example:

go-chromecast tts '<speak>Hello<break time="500ms"/>world.</speak>' \
  --google-service-account /path/to/google-cred.json --ssml

Please consider merging if you like.

[vishen]

Thanks for this @yteraoka! Glad you're enjoying to tool :)

[yteraoka]

Thank you.