allow to supply custom isSqlQuery helper or use better library

Toying with the plugin, I'm coming across valid SQL queries that aren't recognized, such as:

```sql
SELECT * FROM article a WHERE NOT a.archived

-- or

SELECT * FROM article a WHERE (a.authz & 8)::bool
```

One solution would be to either allow users to supply their own SQL parser (though I haven't checked if eslint allows to pass functions as `settings` values in `.eslintrc.js`), or replace the [sql-parse](https://github.com/invercity/sql-parse), which seems unmaintained, with something else.