Add new API parameter "interval" for coin market-charts

[lufrey]

I believe this is quite new, because the documentation is not fully complete yet, but you can add a query parameter "interval" to the endpoint /coins/{id}/market_chart. With that you can set the granularity of the historic data.
You can find it here: https://www.coingecko.com/en/api/documentation
The change required to support this feature is minimal. It would be great if it was supported.
I will submit a Pull Request to update the repository.