This requires a default environment. The state variables are pretty
straightforward. The env variables are a little trickier. I'm just
making most of them empty for now. Note that some of them (like
defaultReferenceDocx/ODT) will be coming out soon anyway.
Until we fix fetchItem and fetchItem' to make use of MonadError, we have
to thow an exception. We'll throw PandocFileReadError. Note that this is
temporary.
