prosopopee: add ratio property for images

To prepare for an alignment fix that requires to know the ratio of the
image, let's add a ratio property.

This property is calling `gm identify` which is already a dependency of
prosopopee.

The output of this particular `gm identify` returns the width and height
separated by a comma.

Signed-off-by: Quentin Schulz <foss@0leil.net>

prosopopee/prosopopee.py

@@ -16,6 +16,7 @@ Options:
 import os
 import shutil
 import socketserver
+import subprocess
 import http.server
 
 import ruamel.yaml as yaml
@@ -266,6 +267,13 @@ class Image(object):
 
         return thumbnail_name
 
+    @property
+    def ratio(self):
+        command = "gm identify -format %w,%h " + self.base_dir.joinpath(self.name)
+        out = subprocess.check_output(command.split())
+        width,height = out.split(',')
+        return float(width) / int(height)
+
     def __repr__(self):
         return self.name