Make Brightness::custom public

[bugadani]

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

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

PR description

cc #148