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 related 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 r0man/sablono#175
• 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

There are problems with this change currently,

Running build with boot devcards shows lots of warnings:

WARNING: No such namespace: react, could not locate react.cljs, react.cljc, or JavaScript source providing "react" at line 6 src/devcards/om/devcards/shared_fn_test.cljs

And errors at browser console, caused by code generated by defui:

[petterik]

@Deraen the CircleCI build failed because it couldn't find artifact devcards:devcards-0.3.0-SNAPSHOT.
Maybe try rebuilding the build? Or if it actually doesn't exist, update the version?

[Deraen]

@petterik It doesn't exist as that depends on devcards update (PR linked), and all the changes are still WIP, and the devcards update depends on om update. It will be fun to release these, when this is ready, due to cyclic dependencies :)

[petterik]

Btw @Deraen, I tried to get om.next to work with react from node_modules and started from your WIP commit in this pull request. I had to some more changes to do with om.dom marcos referring to js/React and js/React.DOM, and maybe something else.

Not everything I did is related to this PR, but here's the diff:
Deraen/om@new-react-require...petterik:petter/new-react-require

[Deraen]

@petterik Nice. Perhaps similar macro fixes will help with Sablono and Rum, which I think also had similar problems.