CLI make and gen have different default behaviors

[stephengoldbaum]

Running moprhir-elm make relies on the morphir.json configuration file by default. Running morphir-elm gen defaults to expecting a pipe and takes the configuration file as an optional argument. It should probably match the make command for consistency and leave the piping as an option.

[AttilaMihaly]

There are two JSON files at play here:

• The morphir.json that morphir-elm make expects is a descriptor that the Elm frontend uses to be able to find the Elm sources and to add extra package information.
• The JSON that morphir-elm make outputs and morphir-elm gen expects as input is the Morphir IR. They use stdout/stdin by default and they have -o and -i options to specify the a file if that's what the user prefers.

Considering the above I think the behavior is consistent but I understand how it can be confusing. To simplify I suggest removing the stdout/stdin path and make the -o/-i options required. I'll also add note to the docs to further clarify the purpose of each JSON.