feat: add option to use single header for destination headers

[gagasdiv]

Single destination headers are used in compliance with RFC 5322 (https://www.rfc-editor.org/rfc/rfc5322#section-3.6.3), which Gmail SMTP complained about. This affects the following headers:

• To
• Cc

[valord577]

@gagasdiv Excuse me, why is this pr closed?

[gagasdiv]

@gagasdiv Excuse me, why is this pr closed?

Ah, sorry, I meant to make this PR to the main branch in my own fork because I needed the changes real quick (I made it to your main branch by mistake).

I can restore the branch and reopen this PR if you want to implement this upstream.
Might note that I'm new to Go and don't know if my changes are up to standards.

Oh, and I like your library! Using it is quite easy, I was looking for a simple Go email library and this fits my project's needs quite well.

[valord577]

It's so kind of you. I hope to restore this PR.

According to this closed PR:

• As code quality, if you restore the PR, I'll make small changes to your code.
• For testing, you can send an email to the address provided by mail-tester, later it will give the report.

[codecov-commenter]

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[valord577]

@gagasdiv Thank you very much again!