Export init fns so advanced optimisations work

[clumsyjedi]

@prakhar1989

You can fork this project and send me a PR for this issue if you're still interested in patching the advanced optimisations issue. As mentioned here: clumsyjedi/lein-chromebuild#7

The thing that needs to happen for advanced optimisations to work is the addition of the ^:export metadata to the init fns. So if you edit background.cljs, new_tab.cljs, content.cljs and popup.cljs and add that, I'll push a new release version. After that all extensions created with lein new chrome-extension will work with advanced optimisations. https://github.com/clumsyjedi/lein-chrome-extension/tree/master/src/leiningen/new/chrome_extension