Support for Multiple Home Assistant Instances

[june128]

I use multiple Home Assistant instances regularly - I have one at home, one at my parents place, where I need to help out once in a while and one at my local hackspace. Therefore it would be nice, if the app supported multiple instances - just like the iOS (and probably Android and macOS) app does. This might also give more purpose to the header bar, if one would implement the multi-instance support using tabs in there (see #10).