decompress applies filter after fully reading all files into memory, and documentation is misleading

[glasser]

The documentation of the filter option is misleading:

Filter out files before extracting

That's not really true. When you use decompress (say, with decompress-tar or decompress-unzip), it fully decompresses the entire tarball into memory before applying the filter. If you're not using the output feature, the filter option is entirely equivalent to just using Array.filter yourself on the return value.

It would make a lot more sense to me if decompress plugins ran filter as soon as they started processing a file, and dropped data from filtered-out files instead of filling RAM with garbage.

I recognize that this is a backwards-compatible change (as currently filter callbacks have access to data) so probably this means that a new "pre-filter" option should be added.

#57 is a PR to add documentation, though I'd suggest fixing the issue instead of documenting it.