Add some site statistics to global context

[leingang]

This is a tiny plugin I wrote to add some keys to the global context.

• post_count: the number of posts in the site
• cat_count: the number of categories in the site
• tag_count: the number of tags in the site.

[leingang]

Oh, OK. I think I might just adapt sidebar, then.

[felixfontein]

I prepared an example on how to use the sidebar plugin with filetreesubs here: https://github.com/felixfontein/filetreesubs-nikola-demo