add keyword NANOSECOND

[waitingkuo]

close #748

let ast = Parser::parse_sql(&dialect, "SELECT EXTRACT(NANOSECOND FROM NOW());").unwrap();
println!("AST: {:?}", ast);

AST: [Query(Query { with: None, body: Select(Select { distinct: false, top: None, projection: [UnnamedExpr(Extract { field: Nanosecond, expr: Function(Function { name: ObjectName([Ident { value: "NOW", quote_style: None }]), args: [], over: None, distinct: false, special: false }) })], into: None, from: [], lateral_views: [], selection: None, group_by: [], cluster_by: [], distribute_by: [], sort_by: [], having: None, qualify: None }), order_by: [], limit: None, offset: None, fetch: None, lock: None })]

[waitingkuo]

add new keywords (follows the ascending order)

[waitingkuo]

add new DateTimeFields

[waitingkuo]

add test cases

[coveralls]

Pull Request Test Coverage Report for Build 3628501076

• 2 of 6 (33.33%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 86.287%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/value.rs	1	2	50.0%
src/parser.rs	1	4	25.0%

Totals
Change from base Build 3623452535:	-0.02%
Covered Lines:	12534
Relevant Lines:	14526

---

💛 - Coveralls

[alamb]

Thanks @waitingkuo