Implemented ScanStream

This function is the basic interface to the word frequency program. It
takes an std::basic_istream to read from, and a pointer to a map as an
output parameter.
Standard input parsing & program output

The program now parses data from stdin via ScanStream, and writes the
resulting list of frequencies to stdout.
Implemented ScanFiles

ScanFiles is now implemented in terms of ScanStream. The type signature
has changed accordingly.