SMTP settings via url

[eval]

This PR allows the SMTP-settings to be configured via a url, e.g.

Mail.defaults do
  delivery_method :smtp, url: 'smtps://user%40gmail.com:[email protected]'
end

The Travis-builds seemed to fail for reasons unrelated to the code. Fixing this failed so I setup separate builds reachable via the ✅ at https://gitlab.com/eval/mail/tree/smtp-url.

The included smtp-delivery examples for gmail and fastmail were successfully tested.

[eval]

Draft-status removed - feedback welcome!

[mikel]

Nice @eval ! - Please rebase this onto latest master, include a changelog and I'll include it in 2.9

[eval]

Hi @mikel, I updated the PR. I made the following changes:

• url-attributes now take precedence, so it's similar to how one would configure the database in Rails.
• I used different default-settings in case of smtp for localhost and non-localhost.
  This to make targeting a test-setup (e.g. mailcatcher) convenient, but also to require starttls by default for non-localhosts.

I tested the setup using mailcatcher and fastmail.

[eval]

Updated docs&code to bring in line with the new acceptable values for enable_starttls.

[sebbASF]

Sorry, but I am not sufficiently familiar with this part of the code to do a review. Please select someone else.