Run a command-line process with data sent, and retrieve the data.

[Chris Katko]

I want to pipe in string data to a shell/commandline program, 
then retrieve the output. But the documentation I read appears to 
only show usage for 'Files' for stdin/stdout/stderr.

ala something like this:
````D
string input = "hello\nworld";
string output;
runProcess("grep hello", input, output);
assert(output = "hello");
````