Add option to parse lastmod and changefreq and priority

[Nedi11]

[Leibel Hecht]

I would love this!

[Tije Vlam]

i wanted it too so i created a pull request (#135) to offer this solution. Don't know if and how fast it will be accepted. If you need it quick @Nedi11 & @lh15 you could also use my branch (https://github.com/tijevlam/sitemapper/tree/Add-option-to-parse-lastmod-and-changefreq-and-priority) of sitemapper to run your project with.