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).

docs/CONFIGURATION.md

@@ -147,6 +147,7 @@ bar {
 | `position`           | enum   | `top`              | Bar position (`top` or `bottom`)              |
 | `vertical_padding`   | u8     | `5`                | Vertical padding above and below text         |
 | `horizontal_padding` | u8     | `5`                | Horizontal padding between bar edges and text |
+| `time_format`        | string | `%Y-%m-%d %H:%M, %A` | strftime format string for the clock display (an empty string hides the clock) |
 
 ### Margins