Implement understanding of formatting verbs

[mlevesquedion]

Currently, the following can cause a report:

core.Sinkf("%T", source)

There is actually no issue in the above example: the type of a sensitive value is not sensitive.

We can likely borrow some logic from the printf analyzer.

If we want to be especially diligent, we could go through all the formatting verbs and determine how to handle each one. From a cursory glance, I think the relevant ones are %T, and maybe %s and %q. For these two, we could e.g. do some kind of analysis of the appropriate String() function for the corresponding argument.