Add download option

[dvdsk]

• show if an episode is already downloaded
• add play support for downloaded episodes
• add way to cancel download
• add seek method in downloaded file

[dvdsk]

• [ ] Episodes are downloaded to Home/${USER}/Downloads/pods.
• [ ] Do not store in a database weather an episode is downloaded, rather query from the filesystem
• [ ] For now the podcasts episode list will show when an episode is downloaded, later in #16 there will be a list of all downloaded episodes
• [ ] A download can be deleted from the podcasts episode list
• [ ] Show an "icon" indicating an episode is being downloaded