_project.py: Parse the new 'shell' section

This new section informs BuildStream how to launch interactive shells
for the specific project.

These new options are part of the effort to make applications work
better inside a `bst shell` environment, issue #223

Initial options include:

  o Default shell command to run for an interactive shell, this is
    used for interactive debugging sessions, and also if no command
    was specified in `bst shell`

  o List of environment variables to inherit from user environment,
    this is useful to propagate some host stuff into the environment
    for debugging; and only considered when it is not an "isolated"
    shell.

These changes also bump the BST_FORMAT_VERSION