Add `markdown-changed` emit event

This event is helpful for packages that wish to extend the default Markdown previewer via `roaster`.

For example, some MD implementations use the following notation for definition lists:

```
term
: definition
: another definition

another term
and another term
: and a definition for the term
```

A second package could list for the event, then update the contents of the preview window.

/cc @kevinsawicki