benchmark

Flush reporters' output streams after writing a benchmark run (#288)

If a reporter's output stream isn't line-buffered (e.g. it's not writing
to a terminal) then it can be some time before a write to it becomes
visible.

This is problematic if, say, you're wanting to use tail -f to view the
file written to via --benchmark_out. Or if the application crashes,
leaving you with no results.

Addressed by flushing the reporters' output streams whenever we invoke
methods that may write to them.