vis_miss: Display warning message giving extra into on missingness

[njtierney]

Print a warning message that is given when vis_miss is run, giving some details on the missingness

[njtierney]

Currently I print out the % of missingness, not sure if it would make as much sense to print extra warning messages or anything at the moment - if you want to do a test of missing data, you should do a test, not rely on a warning message to tell you do do that.

I guess what I'm saying is that I don't want a visualisation function to have two side effects - 1: producing a plot, 2: providing some warning about missingness structure or other feature.

My reason for this is that it would be hard for the user to extract the tests run out of the plot object, because the plot object doesn't return any extra information.