Allow calling stories from other stories

[vpratfr]

fixes #149

Proposed way to do that is simply to recursively inspect the list of tasks returned by a macro in order to extract the real tasks from it (and if a macro is encountered, unfold its content and append it to the list of tasks).

I found no tests for the command and hence did not write anything specific for that.

To test this improvement, one can have the following script:

@task('task-1')
    ln -al
@endtask

@task('task-2')
    pwd
@endtask

@story('story-1')
    task-1
    task-2
@endstory
    
@story('story-2')
    task-1
    task-2
    story-1
@endstory

When running story-1, on should see consecutively task-1, task-2.

When running story-2, on should see consecutively task-1, task-2, task-1, task-2.