Create function `count_missing_values()`

[judith-bourque]

Issue

In a regression, when a independant variable has a missing value, that observation is removed.

It would be nice to be able to count the number of missing values per variable.

Proposed solution

Create function count_missing_values() which wraps around count_na() and count_null().

Function could be applied using mutate(accross(all_of(variables), count_missing_values).

[judith-bourque]

Closing as package no longer actively being developped as of #135.