Json stdout

[drewda]

Hi, here's a PR to add some functionality that is useful for developers chaining this together with other tools on the command line. This is related to at least one issue that has requested some aspects of this in the past. This is my first PR ever for this repo, so please do feel free to revise as needed or to hold if it conflicts with other planned functionality. Thanks.

Summary:

Enables JSON reports to be optionally output directly to stdout for piping to other programs, rather than being written to file system. Log output is suppressed. Involves changing how output directory is handled, but not intended to change existing default functionality in the CLI or the GUI.

closes #1194

Expected behavior:

• Adds an additional CLI argument
• Should not change GUI behavior
• CLI can now support usage such as:

java -jar cli/build/libs/gtfs-validator-0.1.0-SNAPSHOT-cli.jar -u https://data.trilliumtransit.com/gtfs/caltrain-ca-us/caltrain-ca-us.zip --stdout | jq '.notices[] .code'
"route_short_name_too_long"
"stop_without_stop_time"
"unknown_column"
"unknown_file"

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