Set reasonable defaults for cargo config

This sets `build-std=core` by default and uses the atmega328p as the
default target. Users will still need to specify alternate targets if
desired, but it makes the build process much easier for a default case.

Author: n8henrie

.cargo/config.toml

@@ -0,0 +1,5 @@
+[unstable]
+build-std = [ "core" ]
+
+[build]
+target = "avr-unknown-gnu-atmega328"