Enable different timeformat for displaying vs storage

[xeruf]

Feature Request

Add an optional second timeformat option to the config that is only used for output: display_timeformat

Use Case/Motivation

Recently I added the weekday to my timeformat, because it makes it much easier to discover a past entry.
Unfortunately, this also adds this superfluous information to my jrnl file, which I don't want.

Example Usage

timeformat: '%Y-%m-%d %H:%M'
timeformat_display: '%Y-%m-%d %a %H:%M'

Other information

Might make fixing #630 easier.

[wren]

I think this is a good idea. Thanks for the suggestion!

Having separate options for storage and display is something I'd like to move more towards in general, and the datestamp is a good place to start.

I'm not 100% sure what the key should be, but we can work that out later.