[TW-1681] parser: Cannot filter by words with slashes

_Daniel Shahaf on 2015-09-03T06:17:46Z says:_

```
% task add foo/bar
Created task 1.
% task foo/bar
[task next foo/bar]
Cannot divide real numbers by strings
zsh: exit 2
```

The error message is wrong: `foo` is not a real number.

I expected the filter to match the task I had just added.