Use slug in liquid templates

[fauno]

I needed to use the slug on a layout and nothing was being printed until I realized the attribute isn't available in liquid :)

[parkr]

/cc #113

[ashmaroli]

Superseded by #129

[fauno]

@ashmaroli hi, can i ask why this pr went ignored for three years and then a newer one that implements the same feature gets accepted?

[ashmaroli]

Hi @fauno
Technically yes, both PRs have a common ground.
But mainly, this one was about having a slug attribute exposed via Liquid. Like, rendering value via {{ page.slug }}. But the intention was not clear.. there were no tests to demonstrate the expected results either.

Looking at this again, this is not the same feature as what is on current master..
You are welcome to rebase to master, and add some tests.