Feature Request: On-Demand Files

Currently on-demand file downloading is not implemented. #105 was opened and closed due to inactivity. While I'd love to contribute this and make a PR instead of a feature request, this is a bit too daunting of a task for me. When "download files on-demand" is checked in the client on Windows, the file behavior is as follows.

- All files and folders in OneDrive _**exist**_ in the local OneDrive folder but are empty. There is some sort of hook (described later) that downloads the file before it is opened or otherwise used by any program
- If a file is to be opened or otherwise used (such as copying or moving) by any program and it is not yet downloaded, it will be downloaded before it is used. It will be downloaded in-place (meaning it is not stored in a temporary location, to my knowledge). After it has been downloaded like this, the file stays downloaded and can be viewed/modified as many times as you like up until "Free up space" is clicked
- Files and folders in the OneDrive folder can be right clicked and and one of the right click options is "Always keep on this device". What this does is keep those files/folders stored locally instead of waiting for them to be used as described in the first bullet point
- (This one could be a stretch goal for a later addition) When right clicking on a folder in OneDrive, there is an option listed to "Free up space". What this does is removes any files that were downloaded (**with the exception of files/folders which have "Always keep on this device" checked off**)