feat(gatsbynode): add onlyLanguages option

[thompsonsj]

#33 is a great idea to permit localized URLs.

This is a simpler version that does not require slugs to be defined in react-intl message files (e.g. en.json ...etc).

This change adds the possibility to limit page generation to certain languages only.

For example, if creating pages dynamically (e.g. from a CMS or the MDX plugin like in ), you can pass a parameter through the page context to ensure only one version of the page is created.

It makes sense to me to nest this option inside a gatsbyPluginIntl to allow for further options to be passed in this manner.

const language = `fr`;
createPage({
  path,
  component: template,
  context: {
    id: node.id,
    language,
    gatsbyPluginIntl: {
      onlyLanguages: [language]
    },
  },
});