tmux: Activate extended_keys

With it libtermkey/libtickit[^1] mappings are supported.

[^1] - https://www.leonerd.org.uk/code/libtermkey/
[^2] - https://www.leonerd.org.uk/hacks/fixterms/

.config/tmux/tmux.conf

@@ -1,6 +1,7 @@
 set -g default-terminal "tmux-256color"
 set -g mouse on
 set -g mode-keys vi
+set -g extended_keys always
 
 set -g prompt-history-limit 999999
 set -g history-file "$XDG_DATA_HOME/tmux/tmux_history"