Add Category search feature

[zerasul]

in #17 we introduce Category Metadata attribute to post's metadata, a new feature is to search against post category.

[JuanjoSalvador]

This needs a "category structure".

Categories (and tags) will need a name and a slug, just to make it more userfriendly and easier for the navigator. Maybe using some Jinja2 filters or something else.

Example:

Name: Foo tag
Slug: foo-tag
URL: /tags/foo-tag

Same for categories.