read and use additional :request_options on request!/5

[killerswan]

This allows overriding the default options in configuration, like so:

config :tentacat, request_options: [
    {:ssl, [
        {:cacertfile, "/etc/mysvc/mycert.pem"}
    ]}
]

One possible solution to issue #66. 😇

[coveralls]

Coverage increased (+0.7%) to 84.277% when pulling 65e8c9f on killerswan:req-opts into 7f0914f on edgurgel:master.

[edgurgel]

LGTM. What do you think, @duksis ?

[duksis]

Some suggestions from my side:

1. Add the example snippet from this PR description to documentation - maybe readme
2. Rename extra_options to config_options and extract them to a private function

defp config_options
  Application.get_env(:tentacat, :request_options, [])
end

valuable addition even without implementing my suggestions.
thanks @killerswan 👍