Add public method for setting screen contrast (brightness)

[antonok-edm]

When browsing through the API, I was surprised it's not possible to change the brightness of these displays, but it turns out it just wasn't exposed here yet 😄

I've tested this on my own SH1106 display and it appears to work pretty well. It does look like the driver could be fine-tuned to show even darker content using the precharge period? But I'm not familiar with how that works, and the helpful link @limpkin posted to relevant documentation on that issue appears to be dead. The brightness range is sufficient for my own use case in the meantime though.

[limpkin]

hello @antonok-edm , which doc are you referring to? :)

[antonok-edm]

@limpkin the one you posted on the issue I linked to: olikraus/u8g2#98 (comment)

No worries if you can't find it though - it's been a few years!

[jamwaffles]

Thanks for the PR! I'll test this later when I have some hardware to hand.

In the meantime, can I ask you to add a changelog item to CHANGELOG.md please? Something like the following should be fine:

## [Unreleased] - ReleaseDate

+ ### Added
+ 
+ [#20](https://github.com/jamwaffles/sh1106/pull/20) Add `set_contrast` method to set the display contrast/brightness.

[limpkin]

oh right damn it.... I didn't even copy locally that pdf, which I remember to be really well made

[jamwaffles]

Tested on a physical device with STM32F103 and works great. Just missing the changelog item and I can get this merged in :)

[antonok-edm]

@limpkin Oh well. Thanks for checking!

@jamwaffles I updated the changelog.

[jamwaffles]

Sorry for the delay. This is released in v0.3.2 🎉