Expose the current language to templates

[Fabricio Matte]

All templates (index, post, page and category) should have a global variable indicating in which language it is being rendered.

This, coupled with config's custom properties, will allow for rudimentary i18n using keyed dictionaries. There are many other interesting language-specific use cases that this will cover as well.

As for the naming, I believe lang is a suitable global variable name.

[Jaydson Gomes]

Hey! Thanks for this @UltCombo !