Add BTC executable to pull current BTC->USD rate. Update README accordingly

I'm curious if a executable was *intentionally* left out of the gem – the alias function seems pretty dense when, if you're like me, you're really just looking for a quick way to check the exchange rate. 

Let me know if you have any changes you want me to make on my fork – I've tested it locally and it works great! I'll probably keep it around on my local machine as a quick tool regardless!