Passing custom parameters to token_uri

[mitar]

It seems there is no way to pass custom parameters to token_uri. So I want to do a PKCE flow where a custom parameter should be passed both to the authorize_uri (#180) and when code is obtained and exchanged at token_uri, to token_uri as well.

Looking at code here I am confused, it looks like passed params are not added to req at all. Moreover, I do not see any way to pass params in at all. Is this something which is half-done and has not been tested properly?