Add support for category in page permalinks

[dkarzon]

We use categories in our permalinks because reasons. 🎱

Here are the docs: http://jekyllrb.com/docs/permalinks/

[laedit]

I'm good with the "show only a specific category" but why the limit to 3?
Could you make it more generic?

Also there is no reference to :category1 and other in the jekyll doc, do you want to have a "categoty1/category2" url instead of the current "category1-category2"?

[dkarzon]

Looks like I might have read that doco page wrong but our usage is to only use the first category in the url so the :categories parameter doesn't do what we need.

[dkarzon]

Also, with making it generic. I tried only loading it based on the number of categories on the page but if there are no categories it would not remove the category1 placeholder so it would fail when creating the directory.

[laedit]

I'll try to provide a generic solution, but if I fail I'll merge your PR.
Is that ok for you?