Send email using mailgun

Gets values to send from config.id key; group values by theirsource property. For each source, load email_*source*.hbs Handlebars template, merge with each value, and append results to a string.

For example, the low tide worker saves its results to the lowTide key, and the email_lowTide.hbs template outputs a div for each low tide with the level, date, and time:

<div>
    {{level}} low tide - {{date}} @ {{time}}
</div>

Create an array of objects with source and html properties:

[
    {
        "source": "lowTide",
        "html" : "<div>\n    -0.1 low tide - 03/06 @ 9:00am\n</div>"
    },
    ...
]

Set sources to array of source and html objects, and merge with *config.id*.hbs* to produce a message.

{{#each sources}}
    <h4>{{source}}</h4>
    <div>
        {{{html}}}
    </div>
{{/each}}

If there is something to send, send an email via Handlebars using the API key in config.mailgun.apiKey and domain in config.mailgun.domain.