Email autolink shouldn't be started by HTML tags

[aidantwoods]

Fixes #565

[aidantwoods]

The regex looks a bit huge, but it isn't very complex (the majority is defining character classes), so I don't think it'll be any (much?) worse than the existing regex complexity (open to performance suggestions though, cc: @PhrozenByte)

I think filter_var's email filter doesn't match the definition that CommonMark references (CommonMark uses the HTML5 definition), but I'm not sure of the exact discrepancies.

[PhrozenByte]

The regex was taken from CommonMark, right? It is stricter than I'd write it myself, but since we're trying to be compatible with CommonMark we should use it anyway.

Looks great! 👍

[PhrozenByte]

Just making the regex a little easier to read:

$hostnameLabel = '[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?';
$commonMarkEmail = '[a-zA-Z0-9.!#$%&\'*+\/=?^_`{|}~-]++@' . $hostnameLabel . '(?:\.' . $hostnameLabel . ')*';

[aidantwoods]

The regex was taken from CommonMark, right?

Yup, (though they ripped it straight out the HTML5 spec ;p). The only changes I made were escaping a backslash forwardslash and a made the "username" part of the email possessive (since that character class is mutually exclusive with @ I don't think we need backtracking?).

[PhrozenByte]

Alright. Yeah, no backtracking necessary there.