Convert images to images instead of links

Image links will result in a `img` tag instead of a `a` tag. This
behaviour is disabled by default and can be enabled by adding the
following line to to config file:

convertimages: true

To get access to the config in the convert file, I had to move the
config file inside the gmitohtml namespace. This is specially handy
later on if the config file contains other settings which are useful for
the rest of the codebase.

@@ -1,4 +1,4 @@
-package main
+package gmitohtml

@@ -132,3 +131,1 @@
-			link = append(link, []byte(`</a>`)...)
-			result = append(result, link...)
-			result = append(result, []byte("<br>")...)
+			// If link ends with gif/png/jpg, add a image instead of a link

@@ -135,0 +138,4 @@
+				}
+			}
+
+			if isImage && Config.ConvertImages {