mverse 0.2.1

Minor updates and fixes

• Fixed multiverse_tree() colouring by branch.
• Added label_hjust and label_vjust optional arguments to multiverse_tree().
• Updated add_family_branch() and add_formula_branch() so that they take only one branch at a time. mverse can only save one each of the two branch types.
• Added optional arguments parallel and progress to execute_multiverse(). The arguments are passed to multiverse::execute_multiverse() for parallel processing and progress bar display.
• Avoid redundant execute_multiverse() in summary() functions by extracting model objects from multiverse environments instead of executing the multiverses each time.