Review and merge API breaking changes

[ivy]

There are some open pull requests that make changes to public interfaces:

• Break out SMTP Client Reset Method (Break out SMTP Client Reset Method go-gomail/gomail#79)
• Accept io.Reader for Attach and Embed (Accept io.Reader for Attach and Embed go-gomail/gomail#78) - This is specifically for changing signatures to the more idiomatic Attach/AttachFile and Embed/EmbedFile methods.

@pedromorgan: Could we make a next branch where we can stage these changes for a future v3 release branch?

[ivy]

We should also have a discussion about improving the public API to be more idiomatic/ergonomic.

[arbarlow]

👋 Hi, I wrote #78 but never heard back. Happy to change the methods, contribute and discuss the api methods as well, as I and my company use this package extensively!

[ivy]

@arbarlow Thanks for reaching out! I'd be very interested in a pull request against the next branch to add AttachFile and EmbedFile methods to Message.

[RangelReale]

I am going to need modifying this library too, but I saw that the "next" branch is behind the v2 one, we should send pull requests to v2 or next?

[jkak]

I also need to Accept io.Reader for Attach and Embed (go-gomail#78). but there is no progress in v2 or next branch.