add context-aware method to enable timeout + cancel for connections

[nilslice]

@practice-golang, I couldn't push to your branch, so I've pulled your changes into a branch here. Some minor modifications have been made, but I'd like you to verify (if possible) that the sending still works as expected.

Each context should be passed in per call to SendWithContext so that a new context is used per dial, rather than using a global context at the package level. This isn't a super obvious pattern, and I'm sure your solution would have been ok in most cases! This is just a safer way to guarantee that no goroutines share the same context and end up colliding cancellations or timeouts.

If you can, please test in the sending environment you have been working in and let me know if there are any issues. Thanks for your help!

[practice-golang]

Hello @nilslice.
Ok, I will try it. 😄

[practice-golang]

Because I don't know how to edit codes in other branch, I wrote the codes here.

Example in README.md - If cancel is not needed, _ under-score is ok.

ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
defer cancel()

email.go : 129 - Text in email was not sent.

_, err = msg.Write([]byte("DATA\r\n\r\n" + m.Body + "\r\n"))
// _, err = fmt.Fprint(msg, m.Body)
if err != nil {
	return err
}

Then, it works well.

Thank you @nilslice. 😄

EDIT: time package should be imported in example.