Implement "noNodentExtensions" compiler option

Since acorn >= v4 now supports ES7-compliant async and await, the plugin is not mandatory if the nodent extensions are unused. This option prevents nodent requiring the acorn-es7-plugin and therefore disables the extensions async return, async throw, await anywhere and async getters in classes and objects.

As of v3.0.16, acorn v5 is also supported.