Unfurls the given url, throwing an exception if the url is invalid, returning
nil if the given url is nil or not supported, or a map containing some or all
of the following keys (none of which are mandatory):

  {
    :url           - The url of the resource, according to the server
    :title         - The title of the given url
    :description   - The description of the given url
    :preview-url   - The url of a preview image for the given url
  }

Options are provided as key/value pairs, with any/all of the following keys:

  {
    :follow-redirects    (default: true)     - whether to follow 30x redirects
    :timeout-ms          (default: 1000)     - timeout in ms (used for both the socket and connect timeouts)
    :user-agent          (default: "unfurl") - user agent string to send in the HTTP request
    :max-content-length  (default: 16384)    - maximum length (in bytes) of content to retrieve (using HTTP range requests)
    :proxy-host          (default: nil)      - HTTP proxy hostname
    :proxy-port          (default: nil)      - HTTP proxy port
  }

Thrown exceptions will usually be an ExceptionInfo with the ex-data containing:

  {
    :request  - the details of the HTTP request that was attempted
    :response - the details of the HTTP response that was received (comes directly from clj-http)
  }