Recall that :
template <class delimiter = delimiter<','>, class quote_character = quote_character<'"'>,
class first_row_is_header = first_row_is_header,
class trim_policy = trim_policy::trim_whitespace> class Reader

I am not sure about what is the exact benefit of that.
For instance, delimiter is a parameter for Cell only. quote_character as well.
first_row_is_header is used only once in RowIterator begin()
is_trim_char could well be implemented as a std::string+contains(tested char)

I don't think speed would suffer much, and flexibility would be better.

Any opinion ?