Support for Pimoroni Button Shim

I'd like a plugin similar to the gpio-button-plugin from @ratmandu but with support for the i2c gpio expander used in the pimoroni button shim.

this issue is made before i file my pull request.

Start by locating and reading the gpio-button-plugin referenced in the issue to understand the existing plugin entry point. Then inspect the Pimoroni Button Shim's I2C GPIO expander requirements; done means a comparable plugin supports the shim's button input and is verified by the project's available checks.

Written by the indexing model from the issue text.