Always use multi-part format for transclusions

The Parsoid team has changed their spec to always use multi-part
format even for one-part transclusions. This commit changes VE's
output format to always be multi-part, but still accepts single-part
format on the way in.

Change-Id: I6b3156b0b599ad042bb52d7f53dcdeb1c4a0954a
Merge "Always use multi-part format for transclusions"