Add shared feature for just the common bits

Cargo.toml

@@ -30,6 +30,7 @@ atmega32u4 = []
 atmega64 = []
 attiny85 = []
 attiny88 = []
+shared = []
 rt = ["avr-device-macros"]
 
 [dependencies]

README.md

@@ -20,6 +20,7 @@ Via the feature you can select which chip you want the register specifications f
 * `atmega64`
 * `attiny85`
 * `attiny88`
+* `shared` to only include generic shared code (rarely useful)
 
 ## Build Instructions
 The version on `crates.io` is pre-built.  The following is only necessary when trying to build this crate from source.

src/lib.rs

@@ -98,5 +98,6 @@ pub use crate::devices::attiny88;
     feature = "atmega64",
     feature = "attiny85",
     feature = "attiny88",
+    feature = "shared",
 )))]
 compile_error!("You need to select at least one chip as a feature!");