Allow users to select the netlify-plugin-nextjs they want to use so that breaking changes made to the latest version don't mess up my work

[mannycolon]

Is your feature request related to a problem? Please describe.

Today v3.2.2 cause all new deploys to fail (see here: #267) because deploys use the latest version, if we could choose what version to use this could mitigate these kinds of problems.

Describe the solution you'd like
Allow users to choose what version to use.

[lindsaylevine]

https://github.com/netlify/netlify-plugin-nextjs#faq this is answered here now!

[lindsaylevine]

in case that link ever changes:

A: Check out the plugin's releases. You can manually install npm install --save @netlify/plugin-nextjs@3.x.x at the last working version. Once a new working release is out, you can uninstall the plugin at this locked version (npm uninstall --save @netlify/plugin-nextjs), which will re-enable Netlify's build system to use the latest plugin version by default.