All "before" scripts should source ~/.config/OnDemand/${ONDEMAND_APP}.sh, if exists

If we add something like:

```sh
file=${XDG_CONFIG_HOME:-$HOME/.config}/OnDemand/${ONDEMAND_APP}.sh
if [[ -f "${file}" ]]; then
  source "${file}"
fi
```

This allows users to tweak the startup process of each OnDemand app.  This could provide a permanent solution alternative to FR #18.
