Feat/get exchange rate

[Sigularusrex]

It would be great to make fetching rates a little more explicit. If you agree, I'll update the readme too.

For now, tests are failing on my repo

[alexprengere]

Thanks for the idea. I wonder if this is really useful though, as it is basically a partial on c.convert().

[Sigularusrex]

@alexprengere thanks for checking this.

You're right, it makes fetching rates a little more explicit rather than having to derive the conversion rate from
c.convert(1 'EUR', 'NOK')

I think it's a nice quality of life improvement to have a separate function for this, so you can
c.get_exchange_rate('EUR' 'NOK')

But it's your call. lmk if you'd like to continue or feel free to close the PR.