Stream long value

Hi,

Would it be possible to expose a LONG column using a reader interface ? Something like:

``` go
var long mysql.LongText
row.Scan(&long)
long.Read(buf)
```

pgx has something similar for PostgreSQL: https://github.com/jackc/pgx/blob/master/pgproto3/chunkreader.go

What do you think of this ?

Regards,
Étienne