Pass current relative URL into layouts

[dpogue]

This allows access to the relative URL for the currently rendering page. Combined with site data, this could be used to generate a canonical URL meta tag:

<meta name="canonical" content="<%= site.domain + path %>">

Example Outputs

Clean URLs	Source	path value
true	index.md	/
true	articles/first-post.md	articles/first-post/
false	articles/first-post.md	articles/first-post.html