Windows support

Thanks to @xoviat , we have windows support now!

Close stream APIs

Added stream close APIs for Popen object. With this one can use the "send" API and then close the stream so that "wait" wont block. See Issue #21 for more details.

C++17 compatibility

Compiles with C++17 now!
See #17

Read buffer fixed

This release has fix for Issue #16

Major bug fix

ISSUE Reading from a pipe is messed up #14

ISSUE Cannot access process retcode after #13

First release of cpp-subprocess

This is the fist release of cpp-subprocess.