Support reading directly from std::istream

[jwillikers]

I have a use case where I want to parse the first part of a CSV file with my own parser and the rest of it with your parser. Currently, I will have to re-parse most of the file with your parser because it only takes a filename as an argument. I have a std::istream exactly at the position where I want to pick up parsing with your parser. Would it be possible to support reading from a std::istream?