prompt: Add time and user@host.

This is somewhat of a work flow optimization; In the previous prompt,
I've found it too hard to look at previous commands while scrolling
through the terminal.  And every once in while, I'd wanted to eyeball
when I issued a certain command.  Also, when I should share my bashrc
between computers/servers, I'd like to see where I am.
By adding time and user@host in an eye-catching way, I'm killing 2 birds
with one stone; prompt and app/script output is now clearly visually
separated and I can immediately see where I am and at what time the
command was issued.