Twenty Thirteen: add support and styling for Chat structured post format

[lancewillett]

See #23625 for the output Chat post formats will provide, with new the_chat() function, which will replace the_content() in content-chat.php template file.

[lancewillett]

As a proof-of-concept for "striping" rows, colors are wrong but just as an example:

.format-chat .chat-transcript {
	background-color: #e8e5ce;
	margin: 0;
}

.format-chat .chat-transcript dt {
	clear: both;
	color: #686758;
	float: left;
	font-weight: normal;
	padding: 5px;
	width: 40%;
}

.format-chat .chat-transcript dd {
	padding: 5px;
}

.format-chat .chat-transcript cite {
	font-style: normal;
}

.format-chat .chat-transcript dt:nth-of-type(even),
.format-chat .chat-transcript dd:nth-of-type(even) {
	background-color: #f7f5e7;
}

[markjaquith]

In 23806:

Drop the_chat() into twentythirteen. Style updates to follow — just wanted the_chat() to be utilized somewhere.

props lancewillett. see #23869

[lancewillett]

.1 patch (and -joen screenshot) for styles suggested by Joen.

[lancewillett]

In 23817:

Twenty Thirteen: better styles for Chat structured post format output. Props Joen for starter CSS, closes #23869.

[lancewillett]

In 23877:

Twenty Thirteen: update Chat post format styles to match markup changes in r23876. See #23869.