Add support for additional services

[steve228uk]

Currently Fetch only supports Put.io but it could be expanded for additional services

• Dropbox
• FTP

[kylef]

Linking this with #25, perhaps a way to solve this would be using file providers. In theory Fetch app can use the system APIs to view files across all providers. Fetch then wouldn’t need to have any specific support for other services and protocols, it can find other content using system APIs. The Dropbox application provides a file provider, and I think there are other applications that provide FTP/SFTP file providers.

Hopefully the system APIs make this possible to implement.