Add date filtering for SlackReader

[strickvl]

I added the ability to filter slack messages by date.

Context: was hitting API rate limiting fairly often, and also limits to the amount of documents that my index could ingest.

[jerryjliu]

this is great! @strickvl since this is in the core repo atm, could you use gpt_index imports instead of llama_index?

[jerryjliu]

this is great! @strickvl since this is in the core repo atm, could you use gpt_index imports instead of llama_index?

i'll help migrate the reader to llamahub

[jerryjliu]

sweet thanks!