Add support for multiple Mongo aggregation pipeline stages

[emersonf]

The current implementation only allows you to specify a single Mongo pipeline stage, e.g.

in:
  type: mongodb
  aggregation: >
    { 
      $match: { int32_field: { $gte: 5 }}
    }

But the power of Mongo's aggregation pipeline comes from supporting multiple stages, e.g.

in:
  type: mongodb
  aggregation: >
    [
      { 
        $match: {
          int32_field: { $gte: 1 }
        }
      },
      {
        $sort: {
          int32_field: -1
        }
      }
    ]

This PR adds support for multiple stages, keeping backwards compatibility.

[emersonf]

@dmikurube 👋 I don't know how long it typically takes to review and approve PRs. If it can be approved and released quickly, it would avoid me having to set up a private Gem source.

[dmikurube]

@emersonf Thanks for your contribution.

We, as the Embulk team, took over maintenance of this plugin from another individual developer very recently, and we have had little time to learn it so far, unfortunately. Honestly speaking, please expect a certain amount of time to review, merge, and release a new version.