Added SQL parser based on vitess's SQL parser.

Local changes include support for a bit more of the SQL language and
some naming changes to make this package be almost lint free. Note that
there is a remaining bit of lint in the generated sql.go file that I
cannot get rid of because it is generated by yacc (yyEofCode should be
named yyEOFCode).
Add an env variable that allows pre-commit to be skipped.

Only for use in emergencies (e.g. code that files lint that we can't
fix).
Generate sql.go from sql.y.
Merge pull request #69 from petermattis/sqlparser

Sqlparser