Rearranged configuration handling.

[gryf]

Till now, configuration has been done by somehow merging commandline options, reading config file and session files. Design have some flaws, which might occur with unexpected behaviour.

With this patch configuration will be sorted out and following order will be applied:

1. If config file doesn't exists, bandcamp-dl will use defaults and try to write those defaults as a config file.
2. If config does exists, it will be read and options it contain will be applied to the default config.
3. In addition, if anything is passed through commandline, it will be used instead of the default/user configured options.

For example, default config could look as follows:

{
    "base-dir": "/home/username",
    "template": "%{artist}/%{album}/%{track} - %{title}",
    "overwrite": false,
    "no-art": false,
    "embed-art": false,
    "embed-lyrics": false,
    "group": false,
    "no-slugify": false,
    "ok-chars": "-_~",
    "space-char": "-",
    "ascii-only": false,
    "keep-spaces": false,
    "keep-upper": false,
    "no-confirm": false,
    "debug": false
}

user might have in his config file following keys:

{
    "base-dir": "/home/username/music",
    "keep-spaces": true,
    "keep-upper": true,
    "unrelated": "whatever"
}

and invoke a bandcamp-dl with options:

$ bandcamp-dl --base-dir /tmp https://bandcampurl

this in consequence will provide configuration for bandcamp-dl:

{
    "base-dir": "/tmp"
    "template": "%{artist}/%{album}/%{track} - %{title}",
    "overwrite": false,
    "no-art": false,
    "embed-art": false,
    "embed-lyrics": false,
    "group": false,
    "no-slugify": false,
    "ok-chars": "-_~",
    "space-char": "-",
    "ascii-only": false,
    "keep-spaces": true,
    "keep-upper": true,
    "no-confirm": false,
    "debug": false
}

in result, default base-dir will be "/tmp" because commandline options have override config option, keep-spaces and keep-upper have override hardcoded defaults.

Note, that options which are not expected will be ignored (like unrelated key in user config). Invalid json configuration will produce appropriate error message, and such config file will be ignored.

Session files are currently broken and will be subject for following fixes.

[Evolution0]

Works for me, essentially what I had implemented on my end (though not with a class) then losing the ability to connect to bandcamp.com entirely derailed me.

If you want to version bump and switch to a dataclass go ahead, if there is anything else you want to update but are being held back by version just go for it and change the documentation to reflect that.

I'll wait a bit on this PR in case you would like to do that so it can go through as one PR.

[gryf]

It's fine. We can bump Python version and convert class to dataclass in separate PR. It's just internal representation of configuration - it will not affect user config anyway.