Configure global date and time formats

The user can configure their favorite `date_format:` and `time_format:` in their slogger_config as a strftime-string.

This setting can be used in all plugins as follows:

```ruby
Time.now.strftime(@date_format)

Time.now.strftime(@time_format)

Time.now.strftime(@datetime_format) # this is just the conjunction of the first two
```