Feature request: summarise specific variables with specific functions in summarise_draws

Currently summarise_draws applies the summary functions to all variables. In some cases it could be useful to summarise specific variables with specific functions, akin to dplyr:summarise. This would also allow for summary functions to take multiple variables as functions.

A simple example usecase might be calculating the KL-divergence between the distributions of two parameters in the draws object, e.g.:

# draws has variables 'x' and 'y'

# currently doesn't work because it tries to apply the summary function to all variables
summarise_draws(draws, kl_xy = kl_div(x, y))

Start at the summarise_draws entry point and trace how summary functions are applied to variables. Define how variable-specific functions and functions accepting multiple variables should be expressed, using the KL-divergence example as the target behavior. Done means the requested syntax works without applying the function to unrelated variables and is covered by tests.