Support for platform specific dependencies.

Nobody has claimed this yet.

The current version does not seem to store platform specific dependency information (although it seems to parse it just fine).

This is to handle lines like

```
jaraco.windows >= 3.9.1;platform_system=="Windows"
```

currently parsing this line returns an object with the data:
```
{'editable': False,
'extras': [],
'hash': None,
'hash_name': None,
'line': 'jaraco.windows >= 3.9.1;platform_system=="Windows"',
'local_file': False,
'name': 'jaraco.windows',
'path': None,
'revision': None,
'specifier': True,
'specs': [('>=', '3.9.1')],
'subdirectory': None,
'uri': None,
'vcs': None}
```

References on the spec are here:
http://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-platform-specific-dependencies

No file or test is named; start with the parsing path that produces the shown requirement object and trace how the platform marker is handled. Use the jaraco.windows example as the input, and consider the work done when the returned data preserves its platform condition without regressing ordinary requirements.

Written by the indexing model from the issue text.