CLJS-495: add exists? macro

`exists?` can be used to determined whether some value exists. In
JavaScript this is traditionally accomplished with `typeof foo ==
'undefined'`. This allows for feature detection across heterogenous JS
clients.

src/clj/cljs/core.clj

@@ -213,6 +213,9 @@
 (defmacro false? [x]
   (bool-expr (list 'js* "~{} === false" x)))
 
+(defmacro exists? [x]
+  (bool-expr (list 'js* "typeof ~{} !== 'undefined'" x)))
+
 (defmacro undefined? [x]
   (bool-expr (list 'js* "(void 0 === ~{})" x)))