add currency conversion command

[jeremyandrews]

For now I'm primarily interested in comparing USD and EUR, but other currencies should be easy enough to add as well.

For example:

• What is (10) dollars in euros?
• How many dollars can I buy with (100) euros?
• How much does (50) euros cost in dollars?

There's a number of APIs available, I'll start by selecting just one of them:

• https://fixer.io/product
• https://currencylayer.com/product
• https://exchangeratesapi.io/
• https://free.currencyconverterapi.com/

[jeremyandrews]

The amountofmoney entity should simplify implementation:

https://snips-nlu.readthedocs.io/en/latest/builtin_entities.html#amountofmoney