Add /play server endpoint

[Russtopia]

It seemed the server mode was missing an HTTP handler for the play operation which is available in CLI mode. Tested briefly.

Example usage:

$ go-google-home/cmd/ghcli$ ./ghcli -toml default.toml server -port 8084

## Example of sending a 'play' media URL (MP3 file) to ghcli running in server
## mode.

curl --header\
 "Content-Type: application/json
 --request POST\
 --data\
   '{"text": "https://drive.google.com/u/0/uc?id=1RrbBo5BX8XqFpMZgNe_wTehnAUkPSLlB&export=download"}'\
 http://localhost:8084/play

Also works with Google translate's TTS links, which also lets one specify the accent, unlike the /speak endpoint:

$ curl --header "Content-Type: application/json" --request POST --data '{"text": "https://translate.google.com/translate_tts?ie=UTF-8&client=tw-ob&tl=en-uk&q=I+love+coffee"}' http://localhost:8084/play