keyboard_layout: Add format argument

[thiagokokada]

This PR add format argument to keyboard_layout block. For now, it only supports {layout} placeholder.

The reason for this addition is that I want to add an icon to my keyboard block, something like this:

[[block]]
block = "keyboard_layout"
driver = "localebus"
interval = 5
format = " {layout}"

I could have also added an icon to the keyboard block, however I prefer this approach since it is more flexible, and allow in the future support for more information in keyboard_layout block (like keyboard variants).

This is my first code in Rust so if there is any issues, please report it and I will try to fix it 👍 .

[ammgws]

Works for me and could possibly be expanded in the future.

Thanks!

[atheriel]

This should be added to the documentation as well.