Add support of GraphQL variables

[vkublytskyi]

GitHub GraphQL API supports variables that allow specifying of requested data without dynamical change of a query on a client side.

This PR adds the possibility to send variables with a query to GitHub GraphQL API v4.

Usage:

$query = <<<'QUERY'
query showOrganizationInfo (
  $organizationLogin: String!
) {
  organization(login: $organizationLogin) {
    name
    url
  }
}
QUERY;
$variables = [
    'organizationLogin' => 'KnpLabs'
];

$orgInfo = $client->api('graphql')->execute($query, $variables);

[acrobat]

Change the default value to an emtpy array, this way each possible value (user input or default) is the same type

[acrobat]

One small comment, the rest of the PR looks good to me!

[vkublytskyi]

@acrobat, agree that with an empty array as the default value code will be more maintainable in future. Fixed.

[Nyholm]

Cool. It looks good with tests and everything =)

Just add a small note in the change log and we'll be happy to merge.

[vkublytskyi]

Added item to CHANGELOG.md and usage example to doc/graphql.md

[acrobat]

Thanks @vkublytskyi! And congrats on your first contribution on this library!