Faster scraping of Big 5 leagues stats from FBref

New features

Faster scraping of Big 5 leagues stats (by @andrewRowlinson)

FBref has pages for the big five European leagues that allow you to more efficiently get team and player data from multiple leagues. This commit adds a special "Big 5 European Leagues Combined" league option to get data from these pages.

import soccerdata as sd
fbref = sd.FBref(leagues="Big 5 European Leagues Combined", seasons="20-21")
team_season_stats = fbref.read_team_season_stats(stat_type="standard")
player_season_stats = fbref.read_player_season_stats(stat_type="standard")