Add support for ATTACH DATABASE

[lovasoa]

See https://www.sqlite.org/lang_attach.html

Fixes #987

[coveralls]

Pull Request Test Coverage Report for Build 6328708806

• 22 of 26 (84.62%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.003%) to 87.156%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	5	7	71.43%
src/parser/mod.rs	8	10	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/ast/mod.rs	1	78.67%

Totals
Change from base Build 6265241374:	-0.003%
Covered Lines:	16401
Relevant Lines:	18818

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💛 - Coveralls

[lovasoa]

@alamb , can you have a look ? I'd need that for sqlpage.

[alamb]

LGTM -- thank you @lovasoa

[lovasoa]

Thanks !