Report printf specifiers from Service API

Close #174.

The PR seems to cover most important cases: 
 - regular vs. verbatim vs. triple-quote strings
 - single-line vs multiline strings, built-in printf functions
 - 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.