Implement syscall execution mode for long-running interactive commands

We have a few `Syscall.Call*` methods to execute system commands with various background and foreground strategies, but nothing appropriate for long-running commands like builds and test runs where users might want to see and potentially interact with the running command, similar to what [vim-dispatch](https://github.com/tpope/vim-dispatch) and/or [Conque-Shell](https://code.google.com/p/conque/) does.

It might be beyond the scope of maktaba to support something like that depending on how complicated it is to implement. One option would be to build a `Syscall.CallInteractive` interface into maktaba with some kind of basic fallback implementation and provide a way to extend it with other implementations, which would keep the complexity / heavy deps / configuration out of maktaba and offer more flexibility in supporting fancy implementations including existing plugins.