Add [Exposed=*] to expose on all globals

This patch adds support in WebIDL for declaring an interface available
in all contexts.

Closes #468

index.bs

@@ -5963,6 +5963,17 @@ which form (or forms) it is in:
             <code>[Exposed=(Window,Worker)]</code>
         </td>
     </tr>
+    <tr>
+        <td>
+            <emu-nt><a href="#prod-ExtendedAttributeIdentList">ExtendedAttributeWildcard</a></emu-nt>
+        </td>
+        <td>
+            <dfn id="dfn-xattr-wildcard" for="extended attribute" export>takes a wildcard</dfn>
+        </td>
+        <td>
+            <code>[Exposed=*]</code>
+        </td>
+    </tr>
 
 </table>
 
@@ -6024,6 +6035,7 @@ five forms are allowed.
         "="
         "&gt;"
         "?"
+        "*"
         "ByteString"
         "DOMString"
         "FrozenArray"
@@ -6083,6 +6095,11 @@ five forms are allowed.
         identifier "=" identifier
 </pre>
 
+<pre class="grammar" id="prod-ExtendedAttributeWildcard">
+    ExtendedAttributeWildcard :
+        identifier "=" "*"
+</pre>
+
 <pre class="grammar" id="prod-ExtendedAttributeIdentList">
     ExtendedAttributeIdentList :
         identifier "=" "(" IdentifierList ")"
@@ -7984,8 +8001,9 @@ being exposed only on the [=primary global interface=].
 The [{{Exposed}}]
 [=extended attribute=]
 must either
-[=takes an identifier|take an identifier=] or
-[=takes an identifier list|take an identifier list=].
+[=takes an identifier|take an identifier=],
+[=takes an identifier list|take an identifier list=] or
+[=takes a wildcard|take a wildcard=].
 Each of the identifiers mentioned must be
 a [=global name=].
 
@@ -7997,9 +8015,18 @@ defining which global environments the construct can be used in.
 The [=exposure set=]
 for a given construct is defined as follows:
 
-*   If the [{{Exposed}}]
-    [=extended attribute=]
-    is specified on the construct, then the [=exposure set=]
+*   If the [{{Exposed}}] [=extended attribute=] [=takes a wildcard=], then the
+    [=exposure set=] is the set of all interfaces which have a [{{Global}}]
+    or [{{PrimaryGlobal}}] [=extended attribute=].
+    <p class="advisement">
+        <code>[Exposed=*]</code> should be used with care. It is only appropriate
+        when an interface does not expose significant new capabilities. If the
+        interface may be restricted or disabled in some environments, it is
+        recommended to list the globals explicitly.
+    </p>
+*   If the [{{Exposed}}] [=extended attribute=]
+    is specified on the construct with an identifier or identifier list, then
+    the [=exposure set=]
     is the set of all interfaces that have a [=global name=]
     that is listed in the extended attribute's argument.
 *   If the [{{Exposed}}]