fix(source-contentful) add possibility to define search parameters in config

[helloiamlukas]

This will solve #748

Previously the Contentful source only returned the default locale of a field. As mentioned in the Contentful docs, the search parameter locale=* has to be provided to return all locales of a field.

With the changes in this pull request, you can now define search parameters in the plugin configuration.

{
   use: '@gridsome/source-contentful',
   options: {
      space: process.env.CTF_SPACE_ID,
      accessToken: process.env.CTF_ACCESS_TOKEN,
      host: 'cdn.contentful.com',
      environment: 'master',
      typeName: 'Contentful',
      parameters: {
         locale: '*'
      }
   }
}

[hjvedvik]

Looks good now, @helloiamlukas :) Thanks for the PR!

[NicholasRowe]

What is the latest with #1341 ?