Version 6.0.0

• New #101: Add NullMailer and StubMailer (@vjik)
• New #104: Add FileMailer configuration (@vjik)
• New #109: Add HtmlToTextBodyConverter (@vjik)
• New #84: Add MessageInterface methods: getAttachments(), getEmbeddings(), getHeaders(),
  withAddedAttachments(), withAddedEmbeddings() (@vjik)
• New #84: Add Message class that implements MessageInterface (@vjik)
• Chg #100: Remove MessageFactoryInterface and MessageFactory, add MessageSettings instead it (@vjik)
• Chg #101: Remove MessageBodyRenderer and MessageBodyTemplate classes (@vjik)
• Chg #101: Remove yiisoft/view dependency (@vjik)
• Chg #101: Remove MessageInterface methods compose(), withTemplate() and withLocale() (@vjik)
• Chg #102, #108: Rename Mailer to BaseMailer, remove beforeSend() and afterSend() methods, finalize methods,
  and mark dependencies as read only (@vjik)
• Chg #102: Remove getMessage() method from AfterSend and BeforeSend classes, make $message property public
  read only instead it (@vjik)
• Chg #105: Change result type of MessageInterface::with* methods from self to static (@vjik)
• Chg #108: Remove implementation of StoppableEventInterface from BeforeSend and add $preventSendingMessage
  property to it (@vjik)
• Chg #108: Remove beforeSend() and afterSend() messages from BaseMailer (@vjik)
• Chg #84: Rename MessageInterface methods: withEmbedded() to withEmbeddings(), withAttached()
  to withAttachments() and allow passing several files to them (@vjik)
• Chg #94: Remove getError() and withError() methods from MessageInterface (@vjik)
• Chg #94: Change result of MailerInterface::sendMultiple() to SendResults object (@vjik)
• Chg #95: Use new Priority enumeration instead of integer value for define priority in message (@vjik)
• Chg #96: Change order of constructor parameters in Mailer and FailMailer (@vjik)
• Enh #83, #103: Make psr/event-dispatcher dependency optional (@vjik)
• Enh #103: Replace RuntimeException to LogicException when file name callback of FileMailer returns non-string
  value and improve exception message (@vjik)