Add support for getInitialProps in _app.js

[TigranMn]

Goals

1. Reducing server side requests on every page, when all pages need the same data generated on server side
2. Reducing code duplication

Background

This feature is must have, because there is currently no alternative to it. For example i want to generate some data for the schema of the page so that google can read it on every page of the website. In order to do that now i need to make a request in every page's getServerSideProps, instead of making one request in getStaticProps of _app.js

Proposal

The feature need to work like getStaticProps of any other "non-_app.js" page