feat(headers): added flow for custom headers

[mikecabana]

Email setting or query variables will overwrite the custom headers like here

if (isset($from) && !empty($from)) {
    $headers[] = 'From: ' . $from;
} else if (isset($defaultFrom) && !empty($defaultFrom)) {
     $headers[] = 'From: ' . $defaultFrom;
}

Fixes #3

[ashhitch]

Should this be
['type' => ['list_of' => 'String']]
To allow for an array of headers to be sent?

[mikecabana]

It definitely can, I just wanted to keep it simple and use the same logic as the origins since I'm not too savvy in PHP