[TW-1669] annotations: escape the escape character

_Daniel Shahaf on 2015-08-25T07:51:23Z says:_

In 2.4.5 (2331810), it is not possible to add the literal string `\n` (two bytes) to an annotation using `task annotate`:

```
% task 1 annotate 'Lorem \n ipsum \\n dolor \\\n sit
amet'
% task _get 1.annotations.1.description
Lorem
ipsum
dolor \
sit
amet
```

This is a regression from 2.4.4, where the output was:
```
Lorem
ipsum \n dolor \
sit
amet
```

The latter is what I would expect.