WIP: only create on new injector per doc

Previously this branch was creating a new injector for every instance of
an inline tag in every doc.

Inline tags are now defined similarly to normal tags as an object with a
name and associated properties.  Importantly, instead of an injectable
handler function, you now provide an injectable handleFactory, which
should return a handler function of the form:

```
function(doc, tagName, tagDescription) { ... }
```