Make Brightness::custom public

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
[x] 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

cc #148