Add support for customizing internal `egui::style::Style`

[grantshandy]

@RichoDemus / @tqwewe, the default egui look/feel is nice, but from what I can tell this crate doesn't allow changing the style of the console window. If people are going to use this plugin in games I'm guessing they'll want to change the style of the console to fit in with the style of their game's UI.

Simply exposing a public wrapper method over Context::set_style or the console window's Frame (and possibly re-exporting bevy-console's internal egui or bevy-egui) should be enough to let developers make the console look however they want.

Does this seem like something inside the scope of this project? If so, let me know and I'll create a draft PR and work on adding it in the next week or so 👍🏽.

[grantshandy]

Ah, I should have done more research before creating this issue. I was unaware of how bevy's plugin system handles this. Instead I should have been looking into modifying EguiPlugin's Style 🤦🏽. I thought they worked more independently of eachother and EguiPlugin didn't globally control the egui Context.

Thanks for creating this great project 👍🏽.