Add ability to use stdout and stdin instead of /dev/tty

* Report type: Suggestion
* Operating system: Ubuntu 19.10
* Backend used: ncurses (default)
* Cursive version: crates.io 0.14.1

# Problem description

When debugging with [vscode-lldb](https://github.com/vadimcn/vscode-lldb), `/dev/tty` is not available due to the way lldb spawns the debugee. See https://github.com/vadimcn/vscode-lldb/issues/296 for the corresponding ticket.

Since I'd like to be able to launch & debug my cursive application from within vscode, a workaround for that could be to open `stdin` and `stdout` instead of the unavailable `/dev/tty`.

Note that I haven't actually tried whether this will work and fix the problem, so my assumption might be incorrect.