optionally build with egui

optionally build with egui debug interface (enabled by default)

build/run without egui: cargo run --no-default-features

we can write code that is only compiled when the egui feature is enabled by annotating with #[cfg(feature = "egui")] or #[cfg(not(feature = "egui"))]

further reading (with additional links): https://doc.rust-lang.org/cargo/reference/features.html

closes #8 (closed)