Adds a new --max-downloads option

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I added a new --max-downloads command line option to limit the number of files downloaded per job

I wanted to make it easier to stop downloads early without writing an external script or interrupting it manually, and I mainly used it for testing new extractors and previewing the first few files from various sources

The default behavior is unchanged when the option is omitted, and I tested it locally to make sure download halts cleanly after reaching the limit

(this is also my first open source contribution, feedback is very welcome!)

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The only thing I’m not sure about is whether there are any other places where I should update or add documentation

I added the option to gallery_dl/option.py, but I wasn’t sure if there are other files or docs that need changes