Support returning converted string from Pandoc.

[chriskrycho]

This is #7, recreated because I was a dummy and opened that from my master branch instead of an actual feature branch, and so when I updated my master to yours… :cloud: This still addresses half of #5. I'll still be happy to address the other half in a future PR.

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Breaking changes:

• Include UTF-8 conversion errors in the Error type.
• Take OutputKind instead of File as argument to the set_output method, and make OutputKind public.

Non-breaking changes:

• Add a new execute_with_output option.

[chriskrycho]

For sanity's sake, I've assumed #12 here. I'd really like not to do that merge again! :wink:

[oli-obk]

superb! thanks