Improve regexps in `HtmlToTextBodyConverter`

[vjik]

$html = preg_replace('~<(style|script)[^>]*>.*?</\1>~is', '', $html);
// ...

// Does it make sense to remove spaces from the end of lines?
$text = preg_replace("~^[ \t]+|[ \t]+$~m", '', trim($text));

Originally posted by @Tigrov in yiisoft/mailer-view#2 (comment)