Fix: Correctly link images on build

This fixes an issue where images added with the `image` shortcode  are visible via `zola serve` but not `zola build`.

It works by prepending the site's base URL to image names so `http://127.0.0.1:1111/flower.png` and `https://www.awesomesite.com/flower.png` are both valid paths. Right now, only the former is a valid path.

Fixes pawroman#36.

Author: goingforbrooke

templates/shortcodes/image.html

@@ -1,3 +1,3 @@
 {% if src %}
-  <img src="{{ src | safe }}"{% if alt %} alt="{{ alt }}"{% endif %} class="{% if position %}{{ position }}{% else -%} center {%- endif %}" {%- if style %} style="{{ style | safe }}" {%- endif %} />
+  <img src="{{ config.base_url ~ src | safe }}"{% if alt %} alt="{{ alt }}"{% endif %} class="{% if position %}{{ position }}{% else -%} center {%- endif %}" {%- if style %} style="{{ style | safe }}" {%- endif %} />
 {% endif %}