Add an option to show the terminal in a existing window

[xhighway999]

Hi, I am using your library in a project in combination with a framework that abstracts away window creation in Dear Imgui. My problem is, your (awesome) library creates its own window. Maybe you want to add a way to show the terminal as a widget and not in another window. Here are some ideas how this could be done:

1. change the default behavior of terminal:show()
2. add another parameter to terminal::show() (maybe something like bool showInWindow = false
3. add a configuration option somewhere
4. have a separate function for this use case

or maybe I'm just using your library wrong but that's for you to decide :D