correlate(): Add sample size output and comparison variable option

[LKobilke]

Two requests for correlate():

1. Include sample size ('n') in the output. Or give an option to include it (like in SPSS).
2. Allow specifying a comparison variable (= SPSS's "correlate with") to reduce output size. Currently, the output can be overwhelming.

[LKobilke]

1. First request, include n:
By default, correlate() will now include sample size:

And to_correlate_matrix() provides the sample size as a header if only two variables are specified:

2. Second request, correlate with focus variable:
I've implemented a "with" parameter in the correlate() function that takes a focus variable that everything else is correlated against:

Also works without specifying vars:

Specifiyng both the partial and the with paramter in a correlate() will raise an error:

[MarHai]

Great addition!

My only criticism here is, again, on the string format of the with parameter -- the remainder of the package always works with dplyr vocabulary. We should stick to that, I think.