Add sub filter

This is the opposite of the default `add` filter.

tubesync/sync/templatetags/filters.py

@@ -12,3 +12,14 @@ def bytesformat(input):
         return output
     return output[: -1 ] + 'iB'
 
+@register.filter(is_safe=False)
+def sub(value, arg):
+    """Subtract the arg from the value."""
+    try:
+        return int(value) - int(arg)
+    except (ValueError, TypeError):
+        try:
+            return value - arg
+        except Exception:
+            return ""
+