Add Audience#filter method

[astei]

This method would allow a user of the Adventure API to easily synthesize audiences that meet some arbitrary condition, i.e.

audience.filter(this::canSeeAnnouncements).sendMessage(announcement);

This method has many uses that are far too many to enumerate here.

[Narimm]

is this not effectively a duplicate of #154

[astei]

I talked with kashike before filing this issue, and he said your issue is better placed in adventure-platform, not adventure itself.

[Narimm]

Yeah we discussed the differences as well - I just wondered with mine is just not an extension really of yours