Add ability to specify per-file options in Add by URI.

When downloading files where the filename is not part of the URI, it is impossible to rename them without scrolling endlessly until the "out" setting in Advanced Settings. Even this only enables us to download one URI at a time, since Advanced Settings are global to all files being added.

This PR enables the ability to specify options alongside the URIs, with `--` flags. These will override the ones defined in Advanced Settings.

Example:

```
http://ex1.com/f1.jpg http://ex2.com/f1.jpg
http://ex1.com/f2.mp4 http://ex2.com/f2.mp4 --out=file2.mp4
```

The `out=file2.mp4` setting will be passed only for the relevant file, and not for all as would happen if we modified the out parameter in Advanced Settings (which wouldn't work anyway).