Extremes and WITH TOTALS support

[blide]

Currently Iter.Scan() breaks on the first empty line and misses extremes and WITH TOTALS rows which separated from the main data by an empty line. https://clickhouse.yandex/reference_en.html#TabSeparated
I see 2 ways:

1. Include additional data by default
2. Add flag Iter.SetAdditionalData(bool)

[m1nor]

I have no time to implement it right now, but pull requests are welcome, I'll leave this issue open.

[blide]

@m1nor which way would you prefer?

[m1nor]

I think explicit variant is preferable, let's add func (i *Iter) SetOptions(opt IterOptions)