add: -w/--keyword flag for retrieve/askdir + allow selecting multiple datasets

[iwilltry42]

Upstream: philippgille/chromem-go#96

knowledge retrieve and askdir get new/updated flags:

• --keyword / -w:
  • enable Hybrid search by pre-filtering the dataset for documents containing (or not containing) some given keywords
  • use - as a prefix for the word to add it to the "not-include" list
  • included keywords are OR'd, excluded keywords are AND'd
  • Example: knowledge retrieve -w "foo" -w "bar" -w "-spam" -w "-eggs" "some query" will retrieve documents that contain either "foo" or "bar" but definitely not "spam" or "eggs"
    • Same via env: KNOW_RETRIEVE_KEYWORDS="foo,bar,-spam,-eggs" knowledge retrieve "some query"
  • Pitfall: `knowledge retrieve "what is foo?" -w "-foo" will likely return nothing (you don't say :O)
• --dataset / -d can now be used multiple times
  • depending on the used retriever (e.g. the default BasicRetriever won't work with this), multiple selected datasets will be used for retrieval

Fixes #55