Add option to flatpak-spawn to specify the working dir of a command

[mbooth101]

This allows invoking a command in a directory that is not necessarily visible from inside a sandbox

[matthiasclasen]

Makes sense in principle, but we should also update the documentation. For technical reasons, the flatpak-spawn man pages lives in the flatpak doc directory. It is worth explicitly pointing out in the docs that the specified directory needs to make sense in the namespace that the command is running in - either a copy of the sandbox, or the host.

[mbooth101]

For technical reasons, the flatpak-spawn man pages lives in the flatpak doc directory.

Aha, I can submit a separate pull request to that repo to update the docs