IDNA: use proposed UTS46 flags to avoid breaking YouTube

Tests: web-platform-tests/wpt#5976.

Fixes #53 and fixes #267 by no longer breaking on on hyphens in the
3rd and 4th position of a domain label. This is known to break
YouTube: r3---sn-2gb7ln7k.googlevideo.com. This is done by setting
the proposed CheckHyphens flag to false.

Fixes #110 by clarifying that BIDI and CONTEXTJ checks are to be done
by setting the proposed CheckBidi and CheckJoiners flags to true.

Follow-up #313 is filed to remove the proposed bits once Unicode is
updated.

url.bs

@@ -282,10 +282,15 @@ U+005C (\), or U+005D (]).
 <a>domain</a> <var>domain</var>, runs these steps:
 
 <ol>
- <li><p>Let <var>result</var> be the result of running <a abstract-op lt=ToASCII>Unicode ToASCII</a> with
- <i>domain_name</i> set to <var>domain</var>, <i>UseSTD3ASCIIRules</i> set to false,
- <i>processing_option</i> set to <i>Nontransitional_Processing</i>, and <i>VerifyDnsLength</i> set
- to false.
+ <li>
+  <p>Let <var>result</var> be the result of running <a abstract-op lt=ToASCII>Unicode ToASCII</a>
+  with <i>domain_name</i> set to <var>domain</var>, <i>UseSTD3ASCIIRules</i> set to false,
+  <i>CheckHyphens</i> set to false, <i>CheckBidi</i> set to true, <i>CheckJoiners</i> set to true,
+  <i>processing_option</i> set to <i>Nontransitional_Processing</i>, and <i>VerifyDnsLength</i> set
+  to false.
+
+  <p class="XXX">This and <a>domain to Unicode</a> below are based on a proposed revision. See
+  <a href="https://github.com/whatwg/url/issues/313">issue #313</a>.
 
  <li><p>If <var>result</var> is a failure value, <a>validation error</a>, return failure.
 
@@ -297,9 +302,9 @@ U+005C (\), or U+005D (]).
 
 <ol>
  <li><p>Let <var>result</var> be the result of running
- <a abstract-op lt=ToUnicode>Unicode ToUnicode</a> with
- <i>domain_name</i> set to <var>domain</var>,
- <i>UseSTD3ASCIIRules</i> set to false.
+ <a abstract-op lt=ToUnicode>Unicode ToUnicode</a> with <i>domain_name</i> set to <var>domain</var>,
+ <i>CheckHyphens</i> set to false, <i>CheckBidi</i> set to true, <i>CheckJoiners</i> set to true,
+ and <i>UseSTD3ASCIIRules</i> set to false.
 
  <li><p>Signify <a>validation errors</a> for any returned errors, and then, return
  <var>result</var>.
@@ -3151,7 +3156,7 @@ spec: MEDIA-SOURCE; urlPrefix: https://w3c.github.io/media-source/#idl-def-
     type: interface; text: MediaSource
 spec: MEDIACAPTURE-STREAMS; urlPrefix: https://w3c.github.io/mediacapture-main/#idl-def-
     type: interface; text: MediaStream
-spec: UTS46; urlPrefix: http://www.unicode.org/reports/tr46/
+spec: UTS46; urlPrefix: http://www.unicode.org/reports/tr46/proposed.html
     type: abstract-op; text: ToASCII; url: #ToASCII
     type: abstract-op; text: ToUnicode; url: #ToUnicode
 </pre>