Postgres `$` dollar placeholder support.

The library isn't able to parse postgres query with `$` placeholders.

```go
tree, err := sqlparser.Parse("INSERT INTO users (email, name) VALUES ($1, $2)")
if err != nil {
    fmt.Println(err)
    // err = syntax error at position 52 near '$'
}
```

I can see the variable `yyDollar` being used so not sure if this dollar placeholder is supported or not.
