Allow configuration of the Parsoid host

Fixes #48 

Currently, there's not a convenient way to test this, since the mechanism for loading a configuration file lives in *./server.js*, which isn't used during testing.

Before we merge this, let's consider how to unify configuration loading so it's used both by `node server` and by mocha.
This feels like it might not be the right place for this default configuration, since it applies only to a specific handler implementation.  Should it be moved to parsoid.js?