Using flexible attribute in path definition

This is a follow-up from a [discussion on the mailing-list](https://groups.google.com/forum/#!topic/beets-users/cuZ-hvk2prU).

When using flexible attributes in path definition, missing fields are not expanded, meaning that they appear in the result as they were defined in the path. For example :

```
beet ls -f '$test_attr' album:Madame
```

Could return someting like :

```
flexattr_value
$test_attr
$test_attr
flexattr_value
```

The current behavior has the following advantages :
- It provides a way to differentiate between a missing and empty field.
- It makes debugging path templates easier : you can easily see if you misspelled a field rather than just having it disappear.
- Symmetry with Python's built-in templating.

One the other hand, this can be quite annoying as you generally do not want the `$'field_name'` string to appear in your path and missing fields are very common when using flexible attributes.

They seems to be several option to make this simpler :
- create a new construct for path definition %ifdef{field,true,false}
- output an empty string for the missing field, one can always use the already existing %if construct to specify a default value when needed.

Start with the path-definition and flexible-attribute behavior described in the issue, then review the linked mailing-list discussion to understand the unresolved alternatives. Use the shown `beet ls -f '$test_attr' album:Madame` example to compare missing-field behavior and define the expected result before identifying the relevant implementation and tests.

Written by the indexing model from the issue text.