Feature request: make parse function public

[ericmaster]

Problem:

There are cases were we don't want to crawl through all locations from a sitemaps index, so we may want to filter these first before crawling through the sitemap xmls containing the URLs

Solution:

Make the parse function public so we can filter the parsed xml

https://github.com/seantomburke/sitemapper/blob/9f7017b3f1fa4214e6613f6ffe1c5aa63126677e/src/assets/sitemapper.js#L176