feat: enhance email notification to send alerts to multiple recipients

[bbarman4u]

This PR enhances the email notification functionality where email alerts can now take a set of email addresses separated by comma.
This is related to this github issue #171

[jemand771]

after staring at this for a bit, I think the use of FormatAddress narrows down what you can put into statping's email field.
previously, you could put any of these:

willy@jemand771.net
<willy@jemand771.net>
Willy <willy@jemand771.net>

and it would just set the header to that. according to RFC5322, section 3.4, all of those are valid email header contents.

with FormatAddress, users may only put in an email address like willy@jemand771.net, but the "full format" is no longer supported.
not sure if that's realistically gonna be a problem, but I'd rather not make a breaking change for a simple feature.

also see FormatAddress's source - it doesn't even do anything when you pass in an empty name :D

[jemand771]

I haven't tested this at all yet, but I like the suggestion (with some minor changes :b)

[jemand771]

haven't tested this, I'm thinking of something like

Suggested change

	tempSplit := strings.Split(email.To, ",")
	addresses := make([]string, len(tempSplit))
	for i := range addresses {
	addresses[i] = m.FormatAddress(tempSplit[i], "")
	}
	m.SetHeader("To", addresses...)
	addresses := strings.Split(email.To, ",")
	m.SetHeader("To", addresses...)

(see my long comment from earlier)

[bbarman4u]

@jemand771 Thanks for the suggestions. I did get to test your proposed changes as well as the earlier version, both of the cases it does work fine with various email formats, however since your proposed changes are more succinct I agree it makes more sense to use that version. Thank you again for helping to review this.

[bbarman4u]

@jemand771 Please review the updated PR.