Experimental, subject to change.

Needs `postal`, Ref. <https://github.com/drewr/postal>.

Returns a signal handler that:
  - Takes a Telemere signal (map).
  - Sends the signal as an email to specified recipient.

Useful for emailing important alerts to admins, etc.

Default handler dispatch options (override when calling `add-handler!`):
  `:min-level`  - `:info`
  `:rate-limit` -
    [[5  (enc/msecs :mins  1)] ; Max 5  emails in 1  min
     [10 (enc/msecs :mins 15)] ; Max 10 emails in 15 mins
     [15 (enc/msecs :hours 1)] ; Max 15 emails in 1  hour
     [30 (enc/msecs :hours 6)] ; Max 30 emails in 6  hours
     ]

Options:
  `:conn-opts` - Map of connection opts given to `postal/send-message`
    Examples:
      {:host "mail.isp.net",   :user "jsmith",           :pass "a-secret"},
      {:host "smtp.gmail.com", :user "jsmith@gmail.com", :pass "a-secret" :port 587 :tls true},
      {:host "email-smtp.us-east-1.amazonaws.com", :port 587, :tls true,
       :user "AKIAIDTP........", :pass "AikCFhx1P......."}

  `:msg-opts` - Map of message opts given to `postal/send-message`
    Examples:
      {:from "foo@example.com",        :to "bar@example.com"},
      {:from "Alice <foo@example.com", :to "Bob <bar@example.com>"},
      {:from "no-reply@example.com",   :to ["first-responders@example.com",
                                              "devops@example.com"],
       :cc "engineering@example.com"
       :X-MyHeader "A custom header"}

  `:subject-fn`      - (fn [signal]) => email subject string
  `:subject-max-len` - Truncate subjects beyond this length (default 90)

  `:body-fn` - (fn [signal]) => email body content string,
                 see `format-signal-fn` or `pr-signal-fn`

Tips:
  - Ref. <https://github.com/drewr/postal> for more info on `postal` options.
  - Sending emails can be slow, and can incur financial costs!
    Use appropriate handler dispatch options for async handling and rate limiting, etc.