Pretty print datapackage.json descriptors for human readability

[zaneselvans]

We pretty-print the XBRL taxonomy in JSON to enable human readability, but the datapackage.json outputs are still all put on a single line, which is both difficult to read as a human being, and strain some editors, which read only some lines into memory to improve performance.

It seems like it should be easy to pretty-print the datapackage outputs as well.