Add the ability to add the time to the preprompt line

[akirk]

Fixes #667. Happy to receive guidance on how to better follow your naming conventions.

My solution is superior to these:

• With the proposed workaround in Add option to display date/time/datetime/timestamp in pre-prompt line #667, you cannot re-source your .zshrc because it redefines the same function.
• With the solution from the wiki, the time appears before the arrow, not in the preprompt line.