Don't consider text files animations.

Files with file-ending of "txt" are now considered multi-frame
images not displayed as animation. That allows all of them to be
shown and not get stuck in a forever-animation if this is the
only file given on the command line

Issues #68

src/image-display.cc

@@ -161,8 +161,9 @@ bool ImageLoader::LoadAndScale(const DisplayOptions &opts,
     // got back (or is there ?), so we use a blacklist approach here: filenames
     // that are known to be containers for multiple independent images are
     // considered not an animation.
-    const bool could_be_animation =
-        !EndsWith(filename(), "ico") && !EndsWith(filename(), "pdf");
+    const bool could_be_animation = !EndsWith(filename(), "ico") &&
+                                    !EndsWith(filename(), "pdf") &&
+                                    !EndsWith(filename(), "txt");
 
     // We can't remove the offset yet as the coalesceImages() might need images
     // prior to our desired set.