Support for streaming/standard input

[duncandewhurst]

We're using Flatten Tool for flattening in OFDS, which offers JSON Lines as a format for large files.

As far as I can see, currently Flatten Tool only supports file input, so users would need to segment/repackage data published using JSON Lines in order to pass it to Flatten Tool, and would then need to recombine the output afterwards.

How much work would it be for Flatten Tool to support streaming/standard input?

[jpmckinney]

See also #297 and related PR #246.

[Bjwebb]

This should be fairly straightforward, as we just iterate through the JSON file, we don't need it all loaded into memory.