sd html: Implement export of metadata in the single document case.

Change-Id: Icf99c9bcea187e2a3127b97050d6ddb072540b0d
html export: Close the <meta> tags.

Change-Id: Icca58dd1531f2442fe5878ed9524a3611c0601ea
sd html: Unit test for the metadata export.

Change-Id: I1e38e28b57e816a0829d1a56f2ffcb32d8f93718
sd html: Provide some minimal reasonable defaults.

Change-Id: I72b02f7d87cf6075d318c1cc0d7288e27bbe1b9c