Enhanced bst shell configuration and cli options

Some changes to the host-files configuration:

  o Dont require `host-files` to not be directories

    We need to specify directories to mount from `project.conf` after all.

  o Added possibility of specifying optional mounts, to avoid
    meaningless warnings where optional files don't exist on
    the host

Added --mount CLI option to `bst shell`

This allows users to explicitly mount whatever they want into the
sandbox environment for `bst shell`.

This closes issue #274