Add comment author and body to placeholder list

Hi,

We've been using this plugin for an internal blog and it seemed useful to include functionality to allow the comment emails to include the comment body and comment authors in the emails.

My changes are pretty basic, and work well, but there are several things I wasn't sure about (so I'm opening this PR to discuss those).

1. I did basic sanitization on the comment body and author with WP's `sanitize_text_field()`. That serves to remove HTML, etc. but also will remove any sort of formatting, which could be problematic for the comment body as displayed in the email.
2. Currently the comment email subject and body share the same interpolation set. It doesn't make a lot of sense to support the comment body in the subject field. I can split those out so that the are interpolated and defined independently of each other; an alternative would be to simply adjust the admin documentation and mention the `{body}` placeholder only for the email body, rather than for the subject as well. Any advice/preferences?

Additionally, my changes introduce new language strings. I generated an updated `.pot` file with Poedit; it found several strings included in the current `.pot` that aren't used in the source code:

* msgid "\\a\\t"
* msgid "http://wordpress.org/plugins/notify-users-e-mail/"
* msgid "Notification of new posts by e-mail to all users"
* msgid "Valerio Souza, claudiosanches, ThalitaPinheiro"
* msgid "http://valeriosouza.com.br"

Should those be retained in the `.pot`, or are they legacy and no longer used with `__()`? If they are still needed I'll simply keep them in the `.pot` and add my own strings.

Finally, I noticed a typo in one of the English phrases: "to display the coment URL" should be "to display the com*m*ent URL". I can easily update that, but that will change the `msgid` in the `.pot` file, thus breaking translation work already done in other languages. Is it generally acceptable to also update those files to know about the new `msgid` or is there a better process for that.

Thanks in advance for a handy plugin!