Puckfetcher

* Support download_backlog and backlog_limit as global defaults for all subscriptions.
- 'download_backlog' defaults to 'true', 'backlog_limit' defaults to 1.
- These can still be changed per-subscription.
* Add use_title_as_filename option.
- This uses the title of the RSS entry as the filename.
- If this option is set to false, the filename from the RSS feed is used.
- 'false' is the default value (this was the old behavior as well).
- NOTE - currently disabled on Windows, because we can't guarantee the feed title will be a
valid filename on Windows. Will come eventually.
* Fixed some typos in log messages.
* Updated example config to show off global defaults.

* Provide a better command line interface.
- This allows passing commands (load, list, etc.) to the program.
- You can also run 'puckfetcher prompt' to get a prompt to enter commands into.
- Use 'puckfetcher --help' to see the possible commands.
* BREAKING CHANGE - Running 'puckfetcher' with no arguments will no longer work - you must
provide a command.
* Puckfetcher is quieter on the command line by default (still logs everything it's doing).
* Fix some bugs in documentation.

None, reupload because build seemed broken.

* Make absolute path detection OS-independent
* Use user's backlog_limit instead of ignoring it, fix logic around that.