Client setting for "X-FS-Feature-Tag" header

[jimmyz]

Enable the setting of the “X-FS-Feature-Tag” header so that developers can easily test their apps against pending modifications to the API.

For information on the usage of the "X-FS-Feature-Tag", see: https://familysearch.org/developers/docs/api/tree/Pending_Modifications_resource

Perhaps the client configuration would look something like this (assuming a client interface like the one proposed in #24):

$client = new FamilySearch\Client(array(
    'redirectURI'=>'http://localhost:5000/familysearch-auth.php',
    'clientId'=>'a0T3000000BfM3mEAF',
    'collection_uri' => 'https://sandbox.familysearch.org/platform/collections', //default to the sandbox
    'pending_modifications' => ['remove-identity-v2-login','301-on-updates-to-merged-entities','source-ref-query-param-change']
));