Add optional format for explain

[Dandandan]

This is supported by Presto/Athena:

https://prestodb.io/docs/current/sql/explain.html
https://docs.aws.amazon.com/athena/latest/ug/athena-explain-statement.html

And I want to use it for enabling graphviz output in DataFusion.

[coveralls]

Pull Request Test Coverage Report for Build 3125335760

• 29 of 31 (93.55%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 85.609%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	10	12	83.33%

Files with Coverage Reduction	New Missed Lines	%
src/ast/mod.rs	1	77.93%

Totals
Change from base Build 3090626203:	0.02%
Covered Lines:	9780
Relevant Lines:	11424

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💛 - Coveralls

[alamb]

Thanks @Dandandan -- I have noticed a set of PRs piled up for this repo. I hope/plan to clear the backlog in the next few days and make a new release