Add mirror (vertical flip) option

[mryndzionek]

• 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

Added a function for mirroring/flipping the display. Not that elegant, but seems to be working (tested on one display so far). Let me know if this is acceptable.

[jamwaffles]

Thanks for the PR! I'll give this a test on my sample displays when I've got a spare minute :)

[jamwaffles]

Looks good so far, just some changes from my testing.

Also needs a changelog entry, something like:

### Added

- [#161](https://github.com/jamwaffles/ssd1306/pull/161) Added a `set_mirror` method to enable or disable display mirroring.

added after the Unreleased heading.

[jamwaffles]

Apologies, I didn't see your updates until now! Please throw me a comment for any future PRs so I don't forget :). Thanks for the fix!