Support for Vertex Program

[tsung-wei-huang]

The vertex will support running external program (exec) in the release 0.2.2. Example usage is like :

G.vertex().on([](dtc::Vertex& v) { std::cout << "ready for ls\n"; }).program("/bin/ls / -al");

The external program will have DTC_BRIDGES in its environment variable to store the file descriptor mapping for its input/output stream connections with other vertices or so.