Add :assigns option & AssignsPlug

[maltoe]

This patch adds ChromicPDF.AssignsPlug and the :assigns option.

• Add AssignsPlug
• Add :assigns option to print_to_pdf/2 and handle it in PDFOptions
• Remove examples directory

[maltoe]

@andreasknoepfle Thank you so much for the review :purple_heart: Have only a couple questions left from my perspective, maybe you can help some more :)

1. Wondering whether I'm doing something wrong wrt. Plug.Crypto.sign/4 here. I believe the "user secret" aka salt is meant to be kept server-side, not handed out alongside the token. Not sure though. I basically hand it out because I wanted to verify the cookie before contacting the Agent to prevent DoS attacks, but perhaps that's bogus and instead the salt should be kept within the agent? Having second thoughts now... Maybe we can talk about this later