Make this library extensible by adding extensions.

[mayank1513]

We have 2 potential opportunities to intercept the process. 1. Custom initializers, 2. Run custom code before setting the state.

[mayank1513]

Added plugins

-- Increased lib size from 389B to 417B -- increase of 28B for those who don't use plugins
-- You can import useRGSWithPlugins from '/with-plugin' if you need that -- this helps keep core functionality size to minimum