Add configs for `text/x-cross-domain-policy`

Ensure that files marked as `text/x-cross-domain-policy` will:

 * be served compressed¹
   (they are `xml` files, therefore, they compress quite nicely)

 * be served with future expires headers
   (by default the cache time is set to one week as most of the
   time the well-known location `/crossdomain.xml` will be used)

Notes:

  * The configurations for the `text/x-cross-domain-policy`
    media type. are added  mainly due to its high usage³.

  * The `text/x-cross-domain-policy` media type cannot be
    accurately² set from a `.htaccess` file, so that part will
    be left to the user.

  * Since the policy files will usually have the `.xml`
    extension, they will not be served with any of the `HTML`
    document related headers.

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¹ http://www.adobe.com/devnet-docs/acrobatetk/tools/AppSec/xdomain.html#policy-file-host-basics
  https://cloud.githubusercontent.com/assets/1223565/4783742/7458c9b0-5d34-11e4-9ea8-aece6f7ec2cd.png

² https://github.com/h5bp/server-configs-apache/blob/2.10.0/dist/.htaccess#L182-L206

³ e.g.: According to the results from the HTTP Archive from 15.10.2014:

    +----+--------------------+---------------------------------+
    |    | number of requests |        media type               |
    +----+--------------------+---------------------------------+
    | 1  |            7406888 | image/jpeg                      |
    |    |                ... |                             ... |
  * | 21 |              54476 | text/x-cross-domain-policy      |
    |    |                ... |                             ... |
    +----+--------------------+---------------------------------+

  Query used:

    select count(requestid) as number_of_requests,
           mimetype as media_type
    from [httparchive:runs.2014_10_15_requests]
    group by media_type
    order by number_of_requests desc;

  https://www.igvita.com/2013/06/20/http-archive-bigquery-web-performance-answers/