[5.4] Allow a plain text alternative view when using markdown within mailables

[jodiedunlop]

Currently a mailable class which builds a message with the markdown() method, cannot specify an alternate text version:

return $this->subject($this->subject)
            ->markdown('emails.enquiry')
            ->text('emails.enquiry_plain');  // Currently Ignored

In some cases, even when using the features of markdown to generate the html, you still need an alternate text version.

This PR allows you to optionally specify a text() view in conjunction with a markdown() view, meaning the code above will work.