Report printf specifiers from Service API

Close #174.

The PR seems to cover the most important cases: 
 - regular vs. verbatim vs. triple-quote strings
 - single-line vs. multiline strings
 - built-in printf functions vs. printf-like user-defined functions

One thing isn't very nice: ASTs don't preserve information whether a string is regular, verbatim or triple-quote. Consequently, we have to lookup the source string to recover that information and create correct offsets for reporting ranges.
Don't worry about the cost, it's not a factor - I'd say keep it simple and remove the laziness