Pass issue descriptions into templates

[kyeah]

Hi there! Similar to #131, I'm considering doing some custom formatting — in my case, by parsing a PR/issue's description to provide better context into a change.

I'd love to make a small PR to pass the description into the templating function; I'm also curious if you think generalizing this behavior to pass the full item into generate (either as another placeholder pair, or by changing the generate signature) would be a reasonable feature to avoid smaller updates in the future.

Thanks in advance!