Add a hotkey to display current time
**Is your feature request related to a problem? Please describe.**
While in ``centered buffer mode`` (``[SPC]wc``) in full-screen mode, nothing but the content is shown.  It's better to has a hotkey to display current time, so we don't need to take out the cellphone or exit centered mode just to see the clock.

**Describe the solution you'd like**
Have a hotkey to display current time largely in the center of screen.

