Add time_format config for custom strftime strings

This lets the user change to any time format they want in the bar.
As part of this, we also change the bar to re-draw every second (to
allow using seconds in the time format string).

man/beansprout.5.scd

@@ -143,6 +143,11 @@ bar {
 *horizontal_padding* _pixels_
 	Horizontal padding between bar edges and text. (Default: 5)
 
+*time_format* _format_
+	strftime format string for the clock display. Invalid format strings
+	are ignored and the default is used instead. Set to an empty string
+	to hide the clock. (Default: "%Y-%m-%d %H:%M, %A")
+
 The bar also supports *margins* and *anchors* child blocks; see *TAG OVERLAY*
 for their format.