Get a tibble of stats for a given fixture ID
     
    
    Usage
    get_fixture_stats(fixture_id = NULL)
 
     
    
    Arguments
    - fixture_id
 
(optional) a fixture ID(s) to get stats for. If NULL,
the default, then all fixtures' stats are returned.
 
 
    
    Value
    
a dataframe with columns 'id', 'stat', 'team', 'player' and 'value'