Add display invertion

[nicochatzi]

Hi! Thank you for helping out with SSD1306 development! Please:

• [x] Check that you've added documentation to any new methods
• [x] Rebase from master if you're not already up to date
• [ ] Add or modify an example if there are changes to the public API
• [x] Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, Changed, etc)
• [x] Run rustfmt on the project with cargo fmt --all - CI will not pass without this step
• [x] Check that your branch is up to date with master and that CI is passing once the PR is opened

PR description

Add Ssd1306::set_invert() to invert the screen drawing.

[jamwaffles]

This is now released in version 0.8.1 :)