Add Upcase ident in Macro

This feature is for rust-css colors.
see : https://github.com/mozilla-servo/rust-css/pull/22

something like:
```
define_color!(aliceblue, 240, 248, 255);
```
which would expand to:
```
static ALICEBLUE: Color = Color { red: 240, green: 248, blue: 255, alpha: 1.0 };
```

when is use upcaseident!(aliceblue), it can become ALICEBLUE(uppercase).