Map name to base URL for API — football_get

Adding in as potentially may allow package to work over a few APIs

football_get_standing(
  competition_id,
  api = "football-data.org",
  api_token = Sys.getenv("API_FOOTBALL_DATA")
)

Arguments

competition_id	The ID for the competition of interest.
api	Name of API. Currently has `"football-data.org"`
api_token	Your API token

League standings as dataframe.