Support external file links.

README.md

@@ -65,6 +65,24 @@ For security and compatibility reasons Gollum wikis may not contain custom CSS
 or JavaScript. These tags will be stripped from the converted HTML.
 
 
+## BRACKET TAGS
+
+A variety of Gollum tags use a double bracket syntax. For example:
+
+    [[Link]]
+
+Some tags will accept attributes which are separated by pipe symbols. For
+example:
+
+    [[Link|Page Title]]
+
+In all cases, the first thing in the link is what is displayed on the page.
+So, if the tag is an internal wiki link, the first thing in the tag will be
+the link text displayed on the page. If the tag is an embedded image, the
+first thing in the tag will be a path to an image file. Use this trick to
+easily remember which order things should appear in tags.
+
+
 ## PAGE LINKS
 
 To link to another Gollum wiki page, use the Gollum Page Link Tag.
@@ -95,13 +113,14 @@ Here are a few more examples:
     [[モルドール]] -> モルドール.ext
 
 
-## ABSOLUTE VS. RELATIVE PATHS
+## ABSOLUTE VS. RELATIVE VS. EXTERNAL PATH
 
 For Gollum tags that operate on static files (images, PDFs, etc), the paths
-may be referenced as either relative or absolute. Relative paths point to a
+may be referenced as either relative, absolute, or external. Relative paths
-static file relative to the page file within the directory structure of the
+point to a static file relative to the page file within the directory
-Gollum repo (even though after conversion, all page files appear to be top
+structure of the Gollum repo (even though after conversion, all page files
-level). These paths are NOT prefixed with a slash. For example:
+appear to be top level). These paths are NOT prefixed with a slash. For
+example:
 
     gollum.pdf
     docs/diagram.png
@@ -113,6 +132,12 @@ structure. These paths ARE prefixed with a slash. For example:
     /pdfs/gollum.pdf
     /docs/diagram.png
 
+External paths are full URLs. An external path must begin with either
+"http://" or "https://". For example:
+
+    http://example.com/pdfs/gollum.pdf
+    http://example.com/images/diagram.png
+
 All of the examples in this README use relative paths, but you may use
 whatever works best in your situation.
 
@@ -124,8 +149,8 @@ use the Gollum File Link Tag.
 
     [[Gollum|gollum.pdf]]
 
-The first part of the tag is the link text. The filename appears after the
+The first part of the tag is the link text. The path to the file appears after
-pipe.
+the pipe.
 
 
 ## IMAGES

lib/gollum/markup.rb

@@ -182,8 +182,15 @@ module Gollum
       parts = tag.split('|')
       name  = parts[0].strip
       path  = parts[1] && parts[1].strip
+      path  = if path && file = find_file(path)
+        ::File.join @wiki.base_path, file.path
+      elsif path =~ /^https?:\/\/.+(jpg|png|gif|svg|bmp)$/
+        path
+      else
+        nil
+      end
 
-      if name && path && file = find_file(path)
+      if name && path
         %{<a href="#{::File.join @wiki.base_path, file.path}">#{name}</a>}
       else
         nil