Revise validate to use export output code

[jamesaoverton]

@lmcmicu implemented a new robot validate command, based on #387. I've been using this for real work already, and I love it.

One thing this does is output to HTML and Excel tables, with highlighted cells. I want to reuse the new export output code instead. There's no need for validate to export to CSV or TSV, but maybe JSON and other formats would work if they can support comments.