Allow buffer reuse in Encoder

[repnop]

Fixes #72

I think this should be sufficient for my use case, however I did have a question. I originally had a new_set_with_buffer but as self.output() doesn't actually use the self.output for the final encoding when is_set_encoding: true and is mem::take(&mut self.output)'d elsewhere, I felt like it would be pretty useless since the whole point is to allow reusing the buffer space for the final encoding, but this does kind of expose this implementation detail and could cause some confusion since there's a missing function, not entirely sure what a good solution is for this.

Let me know if there's anything else you'd like adjusted.

[XAMPPRocky]

Thank you for your PR, and congrats on your first contribution! 🎉

[XAMPPRocky]

Also congrats on finding an ICE. 🙂

[repnop]

well now, I didn't think my PR would do that much damage 😄