added basenameSeparator configuration option for TranslationCheck

The separator for property file basenames was hardcoded to `_` in the TranslationCheck. The logic could be useful for non-translation files as well, though.In our case, we have multiple property files for different environments, e.g. `app-dev.properties` `app-stage.properties`, etc. These might not use `_` as the separator character. 

This PR adds the configuration parameter `basenameSeparator` which defaults to `_` so there's no breaking changes.