Add temporary loaders

[jlowin]

Add a way to temporarily and conveniently add simple knowledge to a bot, including via plugin. E.g. point a bot at a website, document, or other information source, parse it with an appropriate loader, store that in a temporary vector store, and make it available ot the bot.

"Hey bot, index example.com"
"Done! what do you want to know?"

Would provide a much lower-lift interface to exploration than having to go through formal knowledge processing