Add parameter 'e' (exchange) to method get_historical_price()

[sebastianhaberey]

First off, thanks for this convenient library 👍

If I had one wish it would be to add the parameter 'e' to the method get_historical_price(). It is mentioned on the cryptocompare.com website and enables the caller to select a specific exchange as source for the historical price instead of the default cryptocompare.com aggregate CCCAGG.

[lagerfeuer]

Thank you for your input, this is now added with 9ccc781.
If you want more features, feel free to add them yourself and open a pull request or open another issue, bigger changes might take a while because of my class load.