[feature] Show post last modified date

[Vel-San]

Small addition to the posts layout that shows when was the last edit of the post.

Usage:

Upon creation of a post, add 'last_modified_at' in the header with the date of your edit!

Example: last_modified_at: '2020-05-7 15:18:05'

And you should add your timezone in _config.yml

Example:

timezone: Germany/Berlin

How it looks like: