Support aggregation framework queries in mongoexport

For mongoexport you can use --query to define ad hoc queries when exporting data.

In MongoDB 2.6+ the aggregate command returns a result cursor similar to a normal query. It would be nice to be able to specify aggregation framework results with a command line option as an alternative to --query:

For example:

mongoexport --db test --collection foo --aggregate '[ { "$unwind" : { documentTags : 1 } } ]'