create files with a literal `/` in the name

OPC requires file names like "foo/bar", but jszip seems to interpret those as folder names:

```
zip.file('foo/bar', 'baz');
zip.folder('foo').file('bar', 'baz') // equivalent
```

All of the checks seem to directly look for a slash character.  Ideally something like "foo//bar" or "foo\/bar" or some other expression would allow for a literal slash in the name
