perplexity provider

[geemus]

• first pass at perplexity provider, largely cloned from openai to start
• first pass at adding citations block to output
• first pass at stripping citations before submitting messages

Here is my first pass at adding a perplexity provider which displays the citation block, but strips it before submitting continuation. I'd welcome any/all feedback to improve it, so far it seems to be working pretty well for me.

closes #71

[gsuuon]

LGTM, thanks!