Make Zencoder URLs interpolation pattern configurable

Pass `:url`, `:hls_url` and `:hls_origin_url` from `zencoder_options`
set in initializer to `ZencoderAttachment.default_options`.

Allows to override the interpolation pattern used to construct urls to
Zencoder generated files. Previously only available for the :path
option.
Use hls_origin host in m3u8 playlists
Do not prefix relative URLs with Default Protocol in Zencoder Attachment
Merge pull request #145 from tf/zencoder-urls

Make Zencoder URLs interpolation pattern configurable
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