Support subshells

[codehearts]

stub.sh doesn't work with VAR="$(my_stub)" or my_stub | my_command because appending to the call array updates it in the subshell but not the parent shell.

shypy manages to support this by writing stub calls to a temp file, it seems. That's less efficient than what stub.sh is doing, so maybe this feature should be hidden behind a flag and not default behavior?

[Outer2g]

Hi! I've created a pull request with this feature, since I needed it too: #9

I've implemented kind of a similar solution but instead of using a file I've used a named pipe in hopes that it gets a little bit faster