Add YAML option to explain

Attached patch adds YAML output option to explain:

explain (format YAML) select * from information_schema.columns;

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Greg Sabino Mullane greg@turnstep.com
PGP Key: 0x14964AC8 200908281414
http://biglumber.com/x/web?pk=2529DF6AB8F79407E94445B4BC9B906714964AC8