Excluding variable type spec in dfSummary()

[forstem6]

Hi,

first, thank you very much for this package. For me it is perfect for presenting quick overviews or analyses.

One thing I nearly always tweak is getting rid of the variable type specifications. I found your solution on stackoverflow and it works perfectly.
However, it might be useful to add the functionality as an argument to dfSummary(). Unfortunately, I am not skilled enough to add it as a pull request.

Cheers.