Modify tidy_mixture_density()

[spsanderson]

When distributions are passed to this function there should be an option on how to combine them: for example a parameter of .combination_type = c('add', 'multiply', 'stack', 'subtract'). So under the hood we should get something like the following:

> set.seed(123)
> plot(density(rnorm(50) + rbeta(50, .5 ,.5, 0)))

 set.seed(123)
> plot(density(rnorm(50) * rbeta(50, .5 ,.5, 0)))

> set.seed(123)
> plot(density(rnorm(50) - rbeta(50, .5 ,.5, 0)))

> set.seed(123)
> plot(density(rbind(rnorm(50), rbeta(50, .5 ,.5, 0))))

There should also be an option that is boolean of .cumulative_sum = FALSE where the default is FALSE, when set to TRUE the output of the .combinaiton_type

> set.seed(123)
> plot(density(cumsum(rbind(rnorm(50), rbeta(50, .5 ,.5, 0)))))