Make zenburn the default colorscheme

This commit also adds better support for constant.number in the
syntax files.

runtime/syntax/swift.micro

@@ -16,7 +16,7 @@ color statement "(print)"
 color statement "(init)"
 
 # Numbers
-color constant "([0-9]+)"
+color constant.number "([0-9]+)"
 
 # Standard Types
 color type "\ ((U)?Int(8|16|32|64))"