Implement `current` as an parameter to load_teams()

It currently exists in nflreadr, but if true, it would only show the current teams in the NFL (and not outdated teams like STL).

nflreadr equivalent is located [here](https://nflreadr.nflverse.com/reference/load_teams.html).

Current is defaulted to false, since it does not exist. nflreadr has current defaulted to true, creating opposing results.