add support for .prc files

[Docbroke]

.prc files seems to be same as .mobi and can be supported by adding 2 lines in code in det_ebook_cls(file): segment

    elif MOBISUPPORT and filext == ".prc":
        return Mobi(file)

[wustho]

Thanks so much for suggestions, mate! First time I heard this format... Will try to implement