Declare dependency on com.ginkgostreet.multiform

The extension framework presently doesn't have a way to declare/satisfy extension dependencies. If CiviVolunteer is to depend upon an external extension, at minimum we should:

• document this in README.md
• document this in the extensions directory
• document this in volunteer.xml
• throw a warning if the user doesn't have it installed (via hook_civicrm_enable and hook_civicrm_upgrade)
• degrade gracefully (can we offer a feature-reduced but still functional alternative?)