Listen on multiple categories about golongpoll

[jcuga]

Supporting more than one category in the same subscription request would introduce complexity that I am not able/willing to take on any time soon. I updated the README to call out this limitation.

Perhaps some day I will tackle this, but there is no guarantee.

Doing so would likely involve:

• having an option (in golongpoll.Options) or whether or not multiple categories per subscription is supported. by default it would be disabled for performance.
• updating the HTTP API to return a map of category to []Event. Or keep existing []Event since each event json object does have an event.category field. It would be on clients to group the events by category.
• There is a decent amount of work to do in the internal bookkeeping/logic to support this without creating issues like #19. This is the main reason why I do not foresee having time to tackle this at the moment.