Use the LangOptions from the ASTContext
Allow skipping all signals/slots starting with given prefixes

I don't know how to convert Q_PRIVATE_SLOTS to the new syntax.
Often a prefix is used for these slots (e.g. _k_ in kdelibs) which makes
it easy to skip them and massively reduces the need for manually edits
Improve error message output
Output a summary after the tool has run
More than one source files works again. Fixes #3