Branch-wise lastmod

[JasperVanDenBosch]

Option to add lastmod to sitemap, by annotating content files with the name of the branch in which they have last been modified significantly.

Also refactors read() such that ContentNode takes on more of its own construction.

[JasperVanDenBosch]

@ubdbra001 For the Meadows documentation I was keen to get the lastmod entry in the sitemap. This is a feature where the developer (optionally) annotates the content files with the name of the branch in which they have last been modified significantly. Using GitPython this is then turned into a date at build time. There is a warning if there are changes and we are on a new branch.

If you have time, your feedback is very welcome but if not I fully understand. This doesn't affect the builds for eegmanylabs.org (I ran a build locally).

[ubdbra001]

I'll try and take a look, but I can't guarantee that it'll be any time soon.

[JasperVanDenBosch]

I'd like to add another new feature and I'm worried things might diverge too much. I think I'll merge this now, and if you have input later we can always address that in a new PR; I hope that's ok!