v5.0.0

• Adding allowPlusSign configuration flag that when enabled ignores suffixes
  after a plus sign in email addresses. For example, an email addressed to
• Adding support for a catch all mapping by using "@" as the key.
  jane+reminder@example.com is treated as if addressed to jane@example.com.
• Fixing handling of emails with less common header formats, such as where there
  is no space in the header after the colon and before the value.
• Updating aws-sdk dependency to match AWS Lambda environment.