Add support for displaying load average

[pvanheus]

Description

Add support for showing load average (as part of the CPU display).

Issue

Addresses one part of what is mentioned in #235. Please comment on how display is done.

Type of change

Remove the irrelevant ones:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (if this change causes breakage or new behaviour, please state what)
• Refactoring (a change that doesn't change application functionality)
• Other (something else - please specify)

Test methodology

Tested by running on Linux.

Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

If relevant, all of these platforms should be tested.

Checklist

If relevant, ensure the following have been met:

• Change has been tested to work, and does not cause new breakage unless intended
• Code has been self-reviewed
• Documentation has been added/updated if needed (README, help menu, etc.)
• Passes CI pipeline (clippy check and cargo test check)
• Areas your change affects have been linted using rustfmt (cargo fmt)
• No merge conflicts arise from the change

Other information

[ClementTsang]

Sorry for getting to this so late!

Made some slight changes:

• Disabled on Windows, it does nothing there from what I can tell
• Used heim to avoid an unnecessary sysinfo invocation
• Spaced out the load average a bit better
• Force 2 decimal places to avoid weird shifting

Final look:

Ideally this would be user-toggleable via config/flag and work on basic mod as well, but it's fine, I'll gladly accept it as is.

[ClementTsang]

@all-contributors please add @pvanheus for code

Totally forgot to do this, mb!

[allcontributors]

@ClementTsang

I've put up a pull request to add @pvanheus! 🎉