[suggestion] Add charset config option and embeddedImages method

[Emanuel-Palestino]

Charset Configuration

In the company where I am currently working, the use of accents is important because our emails are written in the Spanish language. I think that the ability to allow changing the charset would make the mailing tool more customizable and internationalizable.

Embedded Images

Often, we need to embed images in emails for design purposes. The existing method of attaching files doesn't fulfill this necessity.

Note

I have already implemented these features in our local project. However, according to the contributing guidelines, I open an issue before opening a pull request.

PS

This is my first contribution to other projects :)