Inject lang into getStaticProps and company

[aralroca]

Closes #92

from now on the lang will be available on getStaticProps, getStaticProps and getServerSideProps.

I'm going to explain it better in the docs for 0.13.0 version. But a simple example:

export function getStaticProps({ lang }){
   return { props: { data: getDataFromLang(lang) } }
}