URL-encode additional item `fields` within generated EXTM3U playlists instead of JSON-encoding them.
This is because JSON-encoding additional fields/attributes made it difficult to parse the `EXTINF` line but using URL-encoding for these values makes parsing easy (because URL-encoded values cannot contain commas, quotation marks and spaces).
I introduced the generation of additional EXTM3U item fields earlier this year and I want to correct that now.
**Design/definition background:**
Unfortunately, I didn't find a clear definition of how additional playlist item attributes should be encoded - apparently there is none.
Given that item URIs within an M3U playlist can be URL-encoded already, defining the values of additional attributes to be URL-encoded is consistent design.
I didn't find examples of additional EXTM3U item attributes in the web where the attribute value contains a comma, space or quotation mark but examples that specified numeric IDs and URLs as attribute values.
Because the URL attribute examples I found didn't contain URL-encoded characters and because it is more readable and unproblematic for parsing, I've let the attribute URL encoding treat `:` and `/` as safe characters.
**Breaking change:**
While this is a breaking change in theory, in practice it is not since afaik all integrations of the smartplaylist plugin's additional EXTM3U item attribute generation feature (beets-webm3u) work with simple attribute values such as the item ID (numeric) whose formatting/encoding is not affected when changing from JSON to URL-encoding.
In other words the change is backward-compatible with the beets-webm3u plugin (which I'll adjust correspondingly after this beets PR was merged).
Allow generating extm3u playlists so that they contain additional item fields such as the `id`.
The feature is required by the mgoltzsche/beets-webm3u plugin (M3U server) to transform playlists using a request based item URI template which may require additional fields such as the `id`, e.g. `beets:library:track;$id`.
External Python packages interfacing beets may want to use an in-memory
beets library instance for testing beets-related code.
The `TestHelper` class is very helpful for this purpose.
Previously `TestHelper` was located in the `test/` directory.
Now it is part of `beets` itself (`beets.test.helper.TestHelper`) and
can be easily imported.
Beets web API already allows remote players to access audio files but it doesn't provide a way to expose the playlists defined using the smartplaylist plugin.
Now the smartplaylist plugin provides an option to generate ID-based item URIs/URLs instead of paths.
Once playlists are generated this way, they can be served using a regular HTTP server such as nginx.
To provide sufficient flexibility for various ways of integrating beets remotely (e.g. beets API, beets API with context path, AURA API, mopidy resource URI, etc), the new option has been defined as a template with an `$id` placeholder (assuming each remote integration requires a different path schema but they all rely on using the beets item `id` as identifier/path segment).
To prevent local path-related plugin configuration from leaking into a HTTP URL-based playlist generation (invoked with CLI option in addition to the local playlists generated into another directory), setting the new option makes the plugin ignore the other path-related options `prefix`, `relative_to`, `forward_slash` and `urlencode`.
Usage examples:
* `beet splupdate --uri-format 'http://beets:8337/item/$id/file'` (for beets web API)
* `beet splupdate --uri-format 'http://beets:8337/aura/tracks/$id/audio'` (for AURA API)
(While it was already possible to generate playlists containing HTTP URLs previously using the `prefix` option, it did not allow to generate ID-based URLs pointing to the beets web API but required to expose the audio files using a web server directly and refer to them using their file system `$path`.)
Relates to #5037
The boolean flags `--extm3u` and `--no-extm3u` are replaced with a string option `--output=m3u|m3u8`.
This reduces the amount of options and allows to evolve the CLI to support more playlist output formats in the future (e.g. JSON) without polluting the CLI at that point.
