Add `role` entity selector option

[williambl]

This PR adds a role entity selector option, as asked for in #40.

Here is an example of usage:

Negation is supported, too:

Multiple negated selections can be used; only one non-negated selection can be used.

Unfortunately, entity selector options are not synced to the client, so a vanilla client will see them as invalid:

However, they still work fine when executed.

[williambl]

It may be worth removing the limitation that only one role can be selected at once, so that the example of @a[player_role=mod, player_role=builder] given in #40 can be done.

This would also eliminate about half of the code of this PR, which would be good.

[williambl]

Although, entity selector options work with and, not or, so that example would only select builders who are also mods. Perhaps some custom syntax will be required, such as @a[role=mod|builder].

[haykam821]

I think that using the same behavior as the team entity selector option would make the most sense.

[williambl]

That's current behaviour, as far as I can tell.