Xiaomi Gateway support play_specify_fm

[slydiman]

It is possible to play any audio source on Xiaomi Gateway
'{"id":1,"method":"play_specify_fm","params":{"id":527782369,"type":0,"url":"http://someserver/somefile.m3u8"}}'

Please consider to apply the following patch
https://github.com/MicroSur/xiaomigateway

Closes #698

[starkillerOG]

@slydiman the patch is not needed anymore, the functionality can be added quite easily.
See this PR: #698
That just needs to be tested and cleaned up, then it can be merged.

If you want you could test the code, clean it up a bit and make a new PR to get it merged.