Feature Request: Add summary output of var_selection

Currently, an object of class `var_selection` prints as a nice table, but it contains much more information. It would be useful to add a `summary.var_selection` method to show a little more info. It could also be useful to have an `autoplot.var_selection` method that returns plots of the lambda values for the shrinkage methods and plots of number of variables against the stepwise metric.