mverse 0.2.1

Minor patches

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.

spe_curve() is simplified with the introduction of spec_table(). It also returns a ggplot object now for further modification of the aesthetics.
Renamed ttest_mverse() to t_test_mverse() to be consistent.

Branch options are named and stored when *_branch objects are defined.
Global variables are declared to avoid variable missing in scope message: “.data_mverse”, “.formula_mverse”, “.family_mverse”, “.model_mverse”.
Fixed compatibility issue with multiverse 0.6.2 update by using multiverse:::Multiverse$new() to reset multiverse environment inside reset_parameters() to be compatible (#54).
Fixed t_test_mverse() documentation example.