[Feature Request] parser.print_error into a variable

[ssokolow]

I just incorporated layout-rs into a custom pulldown-cmark frontend I'm working on (eventually to be the new static site generator for my blog) and I'd really like to be able to route the formatted error through my usual error-handling machinery without having to either maintain a downstream patch or momentarily hijack stdout as if I were setting and then unsetting the current working directory for an inflexible subprocess.

I'll offer up a PR if I get to the point where I don't have any more pressing TODOs before anyone else does.