Made the EmailParser reusable by clearing its internal state

The only way to customize the quote regexes is to use the EmailParser directly rather than using the static facade of the library AFAIK. In a DI context, it is likely that the same parser instance will then be used several times rather than creating a new instance each time. But the ``fragments`` property storing the FragmentDTO objects was not cleared at the end of the parsing, making it impossible to parse 2 emails with the same parser instance.