add a CLI chapter source plug-in

[M4rtinK]

• add a new chapter source plug-in with a simple CLI interaface
  • specify a command that returns a list of chapter paths/urls with some additional information
  • specify a command that gets two arguments - remote path/URL and a local path - which downloads chapter from the remote URL to the given local path
  • monitoring the size of the locally created file could be used for a progress bar
• use-case :
  • a remote server with chapter files, communication encapsulated with SSH
  • the list command returns a list of available chapters (might be based on du ?)
  • the get command downloads the chapater files using scp or rsync
• other use cases:
  • using HTTP in place of SSH/SCP
  • running rsync on a given folder
  • update daemon and event screen update notifications
  • rsync on a whole chapter folder structure ?