Example for running processes in task graph

[roland-wolf]

There were numerous requests to run processes with a dependency graph defined by taskflow. Imagine we had a blocking function which forks a child process and waits for the child to complete - this would be the missing piece to have a graph of processes. The blocking function would also pipe the stdout and stderr of subprocesses to the calling process. This pull request contains such a blocking function to fork a process and an example that demonstrates how this looks in a taskflow context. The concept opens a plethora of new applications. For example gmake like applications could now be expressed in C++ and benefit from C++ tooling. Defined graphs could be introspected and exported to ninja format to use an open task runner...... and so on.