Made Filename for dumped sitemaps configurable

In Some cases the filename of genreated sitemaps collate with other ones, so its nice to have them configurable.
*(see Commitmessage(s) for details.)*


configexample:
-------------------------

```yml
presta_sitemap:
    sitemap_file_prefix: another_sitemap  # results in files named another_sitemap.xml; another_sitemap.<section>.xml
```