Allow setting the number of context lines in diff.conf

kittens/diff/diff.conf

@@ -9,6 +9,9 @@ syntax_aliases pyj:py recipe:py
 # for a list of schemes.
 pygments_style default
 
+# The number of lines of context to show around each change.
+num_context_lines 3
+
 # Colors
 foreground            black
 background            white