Add support for attributes in table captions

Table captions did not actually support attributes so far, which caused
wikitext like this to parse the attribute as text content:

{|
|+ style="color: red" | foo
|}

This patch fixes this and adds better DSR to the caption tag. Bug 43582 should
be fixed too.

With https://gerrit.wikimedia.org/r/#/c/44167/, the following additional tests
are green:

+3 wt2html
+1 wt2wt
+3 html2html
+1 selser (though no change assignments for new tests yet)

Change-Id: I69704cefe31e4c2f720c7f1a2bf4a9b5a9b6835f
Merge "Add support for attributes in table captions"