Allow for Bidirectional Auto-Completion

Completion currently works from the primary anchor point outward. You create a heading, and that heading is then added to the auto-complete list for easy entry as cross-references elsewhere. However, often I find myself working in the other direction---I am writing along, and as I do so I make reference to a topic, object or link that doesn't exist yet. I may as well go ahead and type in the syntax for it then and there, but doing so gives me no benefit once I go to add the element elsewhere in the file.

If this isn't making sense, consider this scenario:

```
Here is where I am typing, and I realise I need [A New Topic] or that this
statement requires[^annotation].

# A New<AUTOCOMPLETE>

[^anno<AUTOCOMPLETE>
```

In this case, imagine the user typing in a linear fashion downward, and when they get to the second and third lines, the AUTOCOMPLETE indicator shows where the software is suggesting a completion, based on used syntax elsewhere in the document.