Currency support

[Former user]

I really, really appreciate this project and currency support would be beneficial for all of us. Up until now I was using Numi, but I felt like it's time to move on to Linux and over there nothing compares to speedcrunch.

Example: "50 usd in gbp", "100usd in (x in gbp)" and so on...

Helpful links: https://gist.github.com/Fluidbyte/2973986 https://fixer.io/

[Helder Correia]

Duplicate of #367.