Implement augmented assignment operators

Add support for +=, -=, *=, and /= operators. This is basically just
syntactic sugar, but it's still nice to have

    a += 1

compiles to the equivalent of

    a = a + 1

with all the same implications of scoping rules.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/token.rs

@@ -21,6 +21,12 @@ pub enum TokenKind {
     Star,
     Slash,
 
+    // Augmented assignment operators
+    PlusEq,
+    MinusEq,
+    StarEq,
+    SlashEq,
+
     // Unary operators (not already covered)
     Bang,