Rename archivebox oneshot -> archivebox snapshot & archivebox extract, expose atomic subtasks for future job queue system

[pirate]

I want each thing that ArchiveBox does internally to be a subcommand exposed directly to users or callable by the Huey job queue system.

It will also have the side effect of making the process tree nicely represent what ArchiveBox is doing internally, and allow users to kill stuck subtasks independently without stopping the entire "add" import.

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The old oneshot will be renamed and joined by a new command to run a single extractor method:

archivebox snapshot

Can be run to snapshot an individual URL into the current directory (runs all extractors by default).

archivebox snapshot --methods=all 'https://example.com/somepage.html'
# creates a subfolder for each extractor method, and an index.html and index.json file in $PWD

This works the same way as oneshot does now, and I'll alias oneshot to the new command so we don't break backwards compatibility.

archivebox extract

This runs an individual extractor method and outputs into the current directory.

archivebox extract --method=PDF --method-args-here 'https://example.com/somepage.html'
# writes output.pdf (and an index.json containing cmd+output for each run) into $PWD using the headless browser

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After the refactor, archivebox add will work by internally enqueuing a job that runs archivebox snapshot ... for each imported URL.
The snapshot job then in turn enqueues a job for each extractor needed on that URL.
Each extractor job then runs archivebox extract --method=... internally to write the output into the final archive directory.