[feature request] PGA: download n most starred

[campoy]

Feedback obtained from this doc.

PGA wouldn't let me grab the top-100 most-starred repositories in language X. So I went to multitool to generate that list.

This seems like an easy thing to add if we kept the number of stars on pga.

I imagine this working in conjunction with the already existing filters, so downloading the top 100 java projects would be something like pga get -l java --top 100.

[bzz]

👍 For this to happen, https://github.com/src-d/datasets/issues/43 for ⭐️ that we have here for a while, must be implemented first.

Similar feedback logged in there from the participants of ml-on-code workshop.