title: Randomly sample groups
body: <p>Given a dataframe <code>df</code> with a column called <code>group</code>, how do you randomly sample <code>k</code> groups from it in dplyr? It should return all rows from <code>k</code> groups (given there are at least <code>k</code> unique values in <code>df$group</code>), and every group in <code>df</code> should be equally likely to be returned.</p>