Page path variable

[archiekennedy]

This adds the {{path}} variable that stores the path to the current page, excluding the filename.

Example

src/pages/about.html           -> 
src/pages/subfolder/about.html -> subfolder/

Use case

Generating canonical tags, for example:

<link rel="canonical" href="https://example.com/{{path}}{{page}}.html">

[archiekennedy]

@DanielRuf can you review this PR? Thanks 😃

[archiekennedy]

@joeworkman can you take a look at these changes?