[url] Define the application/x-www-form-urlencoded MIME type (#85)

HTML has/had this:
```
  <h3><dfn><code>application/x-www-form-urlencoded</code></dfn></h3>

  <p>This registration is for community review and will be submitted to the IESG for review,
  approval, and registration with IANA.</p>

  <!--
   To: ietf-types@iana.org
   Subject: Registration of media type application/x-www-form-urlencoded
  -->

  <dl>
   <dt>Type name:</dt>
   <dd>application</dd>
   <dt>Subtype name:</dt>
   <dd>x-www-form-urlencoded</dd>
   <dt>Required parameters:</dt>
   <dd>No parameters</dd>
   <dt>Optional parameters:</dt>
   <dd>No parameters</dd>
   <dt>Encoding considerations:</dt>
   <dd>7bit (US-ASCII encoding of octets that themselves can be encoding text using any <span>ASCII-compatible encoding</span>)</dd>
   <dt>Security considerations:</dt>
   <dd>
    <p>In isolation, an <code>application/x-www-form-urlencoded</code>
    payload poses no security risks. However, as this type is usually
    used as part of a form submission, all the risks that apply to
    HTML forms need to be considered in the context of this type.</p>
   </dd>
   <dt>Interoperability considerations:</dt>
   <dd>
    Rules for generating and processing
    <code>application/x-www-form-urlencoded</code> payloads are
    defined in this specification.
   </dd>
   <dt>Published specification:</dt>
   <dd>
    This document is the relevant specification.
    Algorithms for <span data-x="application/x-www-form-urlencoded encoding algorithm">encoding</span>
    and <span data-x="application/x-www-form-urlencoded decoding algorithm">decoding</span> are defined.
   </dd>
   <dt>Applications that use this media type:</dt>
   <dd>
    Web browsers and servers.
   </dd>
   <dt>Additional information:</dt>
   <dd>
    <dl>
     <dt>Magic number(s):</dt>
     <dd>There is no reliable mechanism for recognising <code>application/x-www-form-urlencoded</code> payloads.</dd>
     <dt>File extension(s):</dt>
     <dd>Not applicable.</dd>
     <dt>Macintosh file type code(s):</dt>
     <dd>Not applicable.</dd>
    </dl>
   </dd>
   <dt>Person &amp; email address to contact for further information:</dt>
   <dd>Ian Hickson &lt;ian@hixie.ch></dd>
   <dt>Intended usage:</dt>
   <dd>Common</dd>
   <dt>Restrictions on usage:</dt>
   <dd>This type is only intended to be used to describe HTML form
   submission payloads.</dd>
   <dt>Author:</dt>
   <dd>Ian Hickson &lt;ian@hixie.ch></dd>
   <dt>Change controller:</dt>
   <dd>W3C</dd>
  </dl>

  <p>Fragment identifiers have no meaning with the <code>application/x-www-form-urlencoded</code>
  type.</p>
```
None of this seem sreally useful, but I guess we can add a similar section.

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/url/issues/85