WIP: Global-exports to require React and support node modules

[Deraen]

This is similar to reagent-project/reagent#306
https://github.com/reagent-project/reagent/blob/master/CHANGELOG.md#080-alpha1-2017-07-31

I'm opening PR's on other projects also:

• WIP: Global-exports to require React and support node modules tonsky/rum#148
• WIP: Global-exports to require React and support node modules omcljs/om#887
• WIP: Global-exports to require React and support node modules bhauman/devcards#127

It is possible further changes to ClojureScript will make this change (partially) unnecessary: https://dev.clojure.org/jira/browse/CLJS-2331

Build causes lots of warnings:

WARNING: No such namespace: react, could not locate react.cljs, react.cljc, or JavaScript source providing "react" at line 19 target/nodejs/out/devcards/util/edn_renderer.cljs
WARNING: Use of undeclared Var react/createElement at line 19 target/nodejs/out/devcards/util/edn_renderer.cljs

(some of the problems are caused by devcards and om probably)

And running tests on Node fails:

/home/juho/Source/sablono/target/nodejs/out/devcards/system.js:405
goog.inherits(ctor__19477__auto__,React.Component);
                                  ^

ReferenceError: React is not defined
    at /home/juho/Source/sablono/target/nodejs/out/devcards/system.js:405:35

[r0man]

The lastest master requires now react, instead of using js/React. Thanks for the initial work on this.