Improve cropping of rendered element

Before, we would assume that the element was always rendered at x=0/y=0
coordinates and crop out the element from the screenshot based on the
height and width of the element. That wasn't a good assumption, as some
rendered components define vertical and/or horizontal margin. I noticed
that a diff for an element with `margin: 0 auto` (centered) wasn't
snapshotted correctly. That's what led me into including the x and y
position of the element as well.

lib/likadan_runner.rb

@@ -38,7 +38,10 @@
       # Save and crop the screenshot
       driver.save_screenshot(output_file)
       cropped = ChunkyPNG::Image.from_file(output_file)
-      cropped.crop!(0, 0, rendered['width'], rendered['height'])
+      cropped.crop!(rendered['left'],
+                    rendered['top'],
+                    rendered['width'],
+                    rendered['height'])
       cropped.save(output_file)
 
       print "Checking \"#{current['name']}\" at #{width}px... "

lib/public/likadan-runner.js

@@ -66,15 +66,21 @@ window.likadan = {
 
       var width = elem.offsetWidth;
       var height = elem.offsetHeight;
+      var top = elem.offsetTop;
+      var left = elem.offsetLeft;
 
       if (this.currentExample.options.snapshotEntireScreen) {
         width = window.innerWidth;
         height = window.innerHeight;
+        top = 0;
+        left = 0;
       }
       return {
         name: this.currentExample.name,
         width: width,
         height: height,
+        top: top,
+        left: left
       };
     } catch (error) {
       console.error(error);