Dynamic author biographys

Modifications to generate author biographys dynamically, so not with one hardcoded author page.

Author biographys live in content/authors, the `pelican_extended_authors` plugin will inject the biographys into global template context. On the author page and authors list page, the available biography will be injected. Further info on the plugin lives in `plugins/README.rst`

Also includes some minor style fixes.

@xolox: can you peer-review this for me? Please let me know if there's anything wrong, I'll be happy to fix that.