Checking data with assertthat

[fniessen]

Hello.

First, thanks for this very nice package I just discovered.

Second, my question: could you have a parameter or another function so that we could check data quality with this package?

What I mean is that, when checking some property on data (for example, that sex == "M" or "F"), it'd be good to get an answer such as "3 contradictions in 2740 observations", and eventually give the opportunity to list those.