Use FileFinderUser role to look for files that should be scanned

The changes in this pull request basically implement the enhancement requested in issue #1 (except for the `POTFILES`) using the approach suggested in the comments for pull request #10. By default, strings will now be extracted from files below `lib/` and the files in the directory configured for the `ExecDir` plug-in (if loaded). This can be changed in `dist.ini` by specifying the new `finder` attribute for the `LocaleTextDomain` plug-in, which is really flexible thanks to the infrastructure provided by `Dist::Zilla`.