Get btc price with websocket

[lth-elm]

Instead of fetching the btc price only when calls are made we open at the launch of the application a websocket that will run in a thread and regularly update the btc and satoshis price per usd(t) in a global variable so that we gain some response time. Unfortunately coingecko and coinmarketcap doesn't offer websocket solution so I used the binance one but it only refer prices with stablecoins which are mostly usd stablecoins and no other currencies.

Fixes #19