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.

Also: this depends on an [unreleased routeswitch feature](https://github.com/wikimedia/routeswitch/pull/6).  Let's not merge this until it is [released](https://github.com/wikimedia/routeswitch/issues/8).
Better to use

```javascript
if (typeof handler === 'function') { .. }
```