Add command line flags

[ejv2]

Currently, podbit relies upon the newsboat config and its format to parse some important options. This is flimsy and breaks very easily. This also breaks the ability to add custom directory structures using the newsboat's template substitution stuff (%h for example).

I suggest that we add a new config file in {XDG_CONFIG_HOME}/podbit/config which contains these options with a similar config format to newsboat. We can then add a few more options to configure too. The db file could also be moved to this location, as I feel that makes more sense than being in the data path (while keeping backward compatibility if needed).

Possible new config keys

• downloads-dir - works as currently intended but is only the base directory into which downloads are placed. The paths passed from newsboat are still used to find the download location
• delete-played-files - copied from the podboat configuration
• ytdl-flags - custom flags passed to youtube-dl or yt-dlp
• clean-queue - dequeue items after they have been listened to and have expired (i.e remove from the queue file). For people like me with 10M queue files

[ejv2]

I think rather than a config file I will be using command line flags. These are easier to use etc.

If users want to permanently configure the program, they can use a script or an alias to pass the required flags.

[ejv2]

Also, I am going to do away entirely with the idea of a "cache directory" and just load downloads from the path specified in the queue file. This will give the user more flexibility with downloads and also remove the complexity of having to detect it and/or walk through directories recursively when we begin to support the string formatting stuff from newsboat.