Render emphasis nodes

Support 'em' nodes which are typically styled in italics in browsers.

Adding in support of:
* https://gerrit.wikimedia.org/r/#/c/145187/
* https://gerrit.wikimedia.org/r/#/c/145192/

Change-Id: I9d06d1d1562e8de4393f4e7ab30f7615600a1ede
Merge "Render emphasis nodes"
Use `Promise.guard` from `prfun`.

Change-Id: I1fd92004a81a1cc27c19cb5f0a5e41d32c5503be