Use a better return type for Annotate.

This way, when the user uses that func:

```Go
anns, err := syntaxhighlight.Annotate(...)

...

sort.Sort(anns)
```

They will not get this error:

```
# command-line-arguments
./main.go:215: cannot use anns (type []*annotate.Annotation) as type sort.Interface in argument to sort.Sort:
	[]*annotate.Annotation does not implement sort.Interface (missing Len method)
```

Which of course the user can fix by converting the type themselves, but it'd be nicer if the API of `syntaxhighlight` is improved to avoid that problem. Hence this PR.

The `github.com/sourcegraph/annotate` package is already imported anyway, so there's no reason not to use its `Annotations` type.