Don't exit terminal on process end when hidden

[erichlf]

Currently when a window is hidden and the process exits the terminal window also exits. It would be nice if the window persisted until it was closed by the user.

Originally posted by @erichlf in #13 (comment)

[erichlf]

It might be useful to look at the following PR: https://github.com/stevearc/overseer.nvim/pull/72/files