Make zenburn the default colorscheme

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

runtime/syntax/vi.micro

@@ -5,4 +5,5 @@ color statement  "\b([nvxsoilc]?(nore|un)?map|[nvlx]n|[ico]?no|[cilovx][um]|s?un
 color statement  "\b(snor|nun|nm|set|if|endif|let|unlet)\b"
 color statement "[!&=]"
 color constant.string ""(\\.|[^"])*"|'(\\.|[^'])*'"
+color constant.number "\b[0-9]+\b"
 color comment  "(^|[[:space:]])\"[^"]*$"