.mfl_logincookie: Get MFL Login Cookie
Description
Gets login cookie for MFL based on user_name/password
Docs: https://api.myfantasyleague.com/2020/api_info#login_info
Usage
.mfl_logincookie(user_name, password, season)
Arguments
user_name
MFL user_name (as string)
Value
a login cookie, which should be included as a parameter in an httr GET request