Add use array for plugins

Currently, users cannot implement plugins because next-i18next calls .init on the i18n instance, and plugins via .use must be called before .init.

We should add a use array to the next-i18next config, so that we can simply do:

config.use.forEach(x => i18n.use(x))
i18n.init() // Somewhere later on

Note that server/client detection will remain a responsibility of the user. Implementation might look like:

const NextI18Next = require('next-i18next').default

const use = []
if (process.browser) {
  use.push(someBrowserPlugin)
}

module.exports = new NextI18Next({ use })

[isaachinman]

Two liner solution: 14d494f.

We may in the future want to talk about removing the .use calls in create-i18next-client, but in my opinion they are a core part of the value added by using this package in the first place.

@lucasfeliciano, @mshahov, and @kachkaev - I would appreciate your help testing this solution out. I’m going to release v0.8.0 now.

[isaachinman]

@lucasfeliciano I’m already working on it, no need for a PR. I’ll handle core features like this myself for the time being.