Allow override of subfield delimiter

Migrate has a hard-coded subfield delimiter of ':'. This conflicts with my use case of destination field names in an XML structure with namespaces, e.g. 'dcterms:title'. By allowing overriding of the subfield delimiter string, I can have field names containing ':'.

e.g.

class MyXMLMigration extends XMLMigration {

  public function __construct($arguments) {
    parent::__construct($arguments);
    // Override Migration subfield delimiter so we can use ':' in XML field names.
    $this->subfieldDelimiter = '::';
  }
}