Add 'browser', 'module', 'jsnext:main' as special keywords

[matteocng]

'module' and 'jsnext:main': these fields point to the ES2015 modules version of a library, featuring native ES2015 'import' and 'export', instead of the legacy, CommonJS format. They are supported by common bundlers such as Webpack 2 and Rollup.

• https://github.com/webpack/webpack/issues/1979
• https://github.com/rollup/rollup/wiki/pkg.module

'browser': this field "is provided by a module author as a hint to javascript bundlers or component tools when packaging modules for client side use".

• https://github.com/defunctzombie/package-browser-field-spec

[coveralls]

Coverage remained the same at 100.0% when pulling afef6f397ef8e6d6b1f3d20c25ad5e4090afb9a9 on matteocng:add-browser-module-jsnext-keywords into 2a2b007c50bbc9d4975567e906f38c26d4902cbd on Kikobeats:master.

[Kikobeats]

Actually I know browser from npm and browserify build.

Let me study the rest of keywords before merge this, thanks for the PR!

[Kikobeats]

Related: http://2ality.com/2017/04/setting-up-multi-platform-packages.html