Add Negro Leagues To `FangraphsLeague`

[nwinston]

Now that Fangraphs supports negro leagues, we should be able to pull that data through pybaseball. New values need to be added to the FangraphsLeague enum otherwise an exception is thrown.

[BrayanMnz]

Looks good to me. @tjburch

[tjburch]

thanks @nwinston, this will be really good to add.

Do you think you could add comments to all the docs where this might be used? E.g. in team_batting.md could change

`league: String. Either "all", "nl", or "al" for determining whether you want data from one league or both. Defaults to "all", for returning data on all teams. `

to

`league: String. Either "all" for all data, "nl" for the National League, "al" for the American League, or "mnl" for the Negro Leagues. Defaults to "all", for returning data on all teams. `

[nwinston]

Done! @tjburch

[tjburch]

Thanks. LGTM! Thanks a ton. Will give a day for anyone else to make comments and merge after. ( @schorrm )

[schorrm]

Looks amazing! (thanks for tagging me @tjburch) One question though: is there a reason why the param can only be 'mnl'?

[nwinston]

Updated the docs in fangraphs.md that says to look at FangraphsLeague for the full set. I'll include that note in the rest of them as well.

[schorrm]

Anyway, more than LGTM, @tjburch merge whenever you want

[tjburch]

In. Thanks again @nwinston 👍👍

[nwinston]

No problem!