make sitemap subclass index, add to routing

[shukryzablah]

In this PR I fix the existing sitemap plugin.

This fixes these two PRs:

• https://github.com/coleslaw-org/coleslaw/issues/142
• https://github.com/coleslaw-org/coleslaw/issues/84

As mentioned in this comment by @PuercoPop https://github.com/coleslaw-org/coleslaw/issues/142#issuecomment-335061002, the sitemap class is subclassing index. Additionally, the entry for the sitemap has to be added to routing in the config for the plugin to work.

https://user-images.githubusercontent.com/28762146/133953586-08b6464b-afff-4df2-a155-0c7a8aadb5e0.mov

[kingcons]

Hadn't stopped by in a while but this seems like a good fix to me. Thanks for authoring it! I'll merge.