composer.lock: deps update:
https://github.com/tijsverkoyen/CssToInlineStyles/compare/1.2.1...e53f25f6685d167f80b9d51b91b09aa70ea6c1c9
https://github.com/swiftmailer/swiftmailer/compare/v5.0.1...32edc3b0de0fdc1b10f5c4912e8677b3f411a230
PHPUnit tests: email contents put into "fixtures" files instead of hardcoding them – it should make easier to add more test emails
CssInlinerPluginTest: less copy-pasting: private $mailer and create_message()
whitespace fixup