Release 5.1.8

Android

• Calendar.js: Add default accountName to getCreateCalendarOptions and getCalendarOptions

iOS

• Readme: Document Contacts usage description
• Fix XCode warnigns:
  • Set defaults for CALENDAR_USAGE_DESCRIPTION and CONTACTS_USAGE_DESCRIPTION if you didn't set something when adding the plugin.
    • XCode would otherwise complain about an empty calendar or contacts usage description
  • Calendar.m: Use [UIApplication openURL:options:completionHandler:] in openCalendar.
    • [UIApplication openURL:] was used before and is deprecated since iOS 10
  • Calendar.m: Use self-> for eventStore in background thread
    • Fixes warning: Block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended behavior

Common

• Readme: Cleanup formatting and code examples