Add last save timestamp in status

Introduces `continuum-timestamp` and `continuum-timestamp-format`
options.

docs/continuum_status.md

@@ -15,3 +15,20 @@ When running, `#{continuum_status}` will show continuum save interval:
 or if continuous saving is disabled:
 
     Continuum status: off
+
+### Last saved timestamp
+
+Along with the current status, continuum can also display the last session save
+timestamp to indicate when the last automatic session save happened. This can be
+enabled by setting the tmux option `set -g @continuum-timestamp 'on'` in your
+`tmux.conf`. The format of the timestamp follows UNIX `date` format and can be
+set using `continuum-timestamp-option`.
+
+Example usage:
+
+    set -g @continuum-timestamp 'on'  # Enable last save timestamp in status
+    set -g @continuum-timestamp-format '%I:%M %p' # Show date as "HH:MM AM"
+
+When running, `#{continuum_status}` will show continuum last-save timestamp:
+
+    Continuum status: 15 <11:54 AM>

scripts/continuum_status.sh

@@ -12,6 +12,12 @@ print_status() {
 	if [ $save_int -gt 0 ]; then
 		style_wrap="$(get_tmux_option "$status_on_style_wrap_option" "")"
 		status="$save_int"
+		local tstamp_option="$(get_tmux_option "$status_timestamp_option")"
+		local tstamp_format="$(get_tmux_option "$status_timestamp_format_option" "%I:%M %p")"
+		if [ $tstamp_option == "on" ]; then
+		  local tstamp="$(date +"$tstamp_format" -d @$(get_tmux_option "$last_auto_save_option"))"
+			status="$status <$tstamp>"
+		fi
 	else
 		style_wrap="$(get_tmux_option "$status_off_style_wrap_option" "")"
 		status="off"

scripts/variables.sh

@@ -35,6 +35,8 @@ status_script="#($CURRENT_DIR/scripts/continuum_status.sh)"
 status_on_style_wrap_option="@continuum-status-on-wrap-style"   # example value: "#[fg=green]#{value}#[fg=white]"
 status_off_style_wrap_option="@continuum-status-off-wrap-style" # example value: "#[fg=yellow,bold]#{value}#[fg=white,nobold]"
 status_wrap_string="\#{value}"
+status_timestamp_option="@continuum-timestamp"
+status_timestamp_format_option="@continuum-timestamp-format" # example value: "%I:%M %p"
 
 systemd_service_name="tmux.service"
 systemd_unit_file_path="$HOME/.config/systemd/user/${systemd_service_name}"