Convert a currency given an exchange rate

For example:

```elixir
# Knowing that 1 USD is equivalent to 15 ARS...
usd = Money.new(100, :USD)
ars = Money.exchange from: usd, to: :ARS, rate: 15
assert ars == Money.new(1500, :ARS)
````
or
```elixir
Money.rate(:USD, :ARS, 15)
usd = Money.new(100, :USD)
ars = Money.convert usd, to: :ARS
assert ars == Money.new(1500, :ARS)
````