Add relative path support for shortcodes

[opeik]

Hi there,

This PR does what it says on the tin. It lets you link to relative files easily:

{{ figure(src="xkcd-234.png", caption="[Escape artist](https://www.xkcd.com/234)") }}

[pawroman]

Hey, I don't have the time to review this properly right now, but it looks like a great contribution.

@heitorPB could you take a look please?

[heitorPB]

Why is this needed? I have the following snippet working without this patch:

{{ figure(src="nomad-client.png",
          alt="Screenshot of the Nomad Web UI showing the supported task drivers.",
          position="center",
          style="width: 80%",
          caption="Screenshot of the Nomad Web UI showing the supported task
                   drivers. Nomad detected Docker and Podman :)",
          caption_position="center") }}

And the file nomad-client.png on the same dir as the markdown (content/posts/nomad/).

You don't get the image correctly without this patch?

[opeik]

@heitorPB Without this patch I have to specify paths relative to the project root.