Plugins config (highlighter) excluded from Chrome ext, but enabled in cloud reader app

[danielweck]

See: https://github.com/readium/readium-shared-js/commit/ca907787342889bed121b7aa90be256a827c882d

TODO: create a build config that ensures the highlighter plugin is built and deployed to Firebase for the cloud reader, but NOT in the Chrome extension.

https://github.com/readium/readium-shared-js/blob/develop/plugins/plugins.cson

Also, need to figure-out why the build-output folder in readium-shared-js does NOT include vendor plugins, see RJS_PLUGINS_OVERRIDE: https://github.com/readium/readium-shared-js/blob/develop/package.cson#L252 https://github.com/readium/readium-cfi-js/blob/develop/readium-build-tools/pluginsConfigMaker.js#L170