Implement --output in tools/parser

[apstndb]

This PR implements --output flag in tools/parse. It controls output of the command.

# default is not changed
$ go run ./tools/parse/main.go -mode expr "1"                
--- AST
&ast.IntLiteral{
  ValueEnd: 1,
  Base:     10,
  Value:    "1",
}
--- SQL
1

# only ast
$ go run ./tools/parse/main.go -mode expr --output ast "1"    
--- AST
&ast.IntLiteral{
  ValueEnd: 1,
  Base:     10,
  Value:    "1",
}

# only unparse
$ go run ./tools/parse/main.go -mode expr --output unparse "1"
--- SQL
1

• close Control output of tools/parse #243

[makenowjust]

LGTM 👍