Add ability to configure repository clone directory (currently system temp)

[Sam13]

I'm running multi-gitter inside a Docker / VS Code devcontainer environment. It would be nice to configure the directory where multi-gitter clones the repositories. Currently this is the system temporary directory: https://github.com/lindell/multi-gitter/blob/c12fb13b4889e174c684f0fc1ef46b52b5462a67/internal/multigitter/run.go#L202

This would allow mounting a Docker volume and examine the GIT changes in interactive mode with a GIT GUI of choice on the Docker host. Currently I have to map the system temporary directory to the Docker host which is not nice.