More natural language matchers

Related to #87

I'd like to write
```
The path /x/y/z should exist
The path /x/y/z should be a file
The path /x/y/z should have setguid
```

Currently I have to write
```
The path /x/y/z should be exist
The path /x/y/z should be file
The path /x/y/z should has setguid
```

Which is feels awkward considering the otherwise neat English definition.