Support formatted JSON output

[llewelld]

User story: "As a CLI user, it would sometimes be helpful to have outputs generated as prettified JSON, so that I can better understand the structure of the data and to make the outputs easier to interpret."

Related to #22.

In some cases, such as when listing finance rows, it would be convenient for the output to be displayed in pretty-print format (e.g. with structural indentation). Currently the outputs are shown as a big block of text, which can be hard to parse as a human.

This is related to #22 and, in fact, if the output was valid JSON there would be less need for this, because it would be possible to pipe the outputs directly into a JSON formatter. This would in fact be even better, because it would mean that users could tweak the output based on their individual preference.