MongoDB-style aggregation pipeline

[Redsandro]

I'm submitting a...

[ ] Regression
[ ] Bug report
[x] Feature request
[ ] Documentation issue or request
[ ] Support request

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[Feature] MongoDB-style aggregation pipeline

So that we can have nice things like $group (advanced counting).

Mingo does it nicely, although it lacks the performance tweaks and indexes.

[Viatorus]

Thank you for your feature request.

Could you please add a code example (data input, query, expected output) how you would like to use LokiJS with aggregations like $group?