Add JSON structure reporting

[Thomas M. DuBuisson]

Issue

Lack of machine parsable (JSON) output makes MIRAI hard to integrate into CI

Steps to Reproduce

MIRA_FLAGS="--json" cargo mirai

Expected Behavior

stdout or a specified reports file in JSON

Actual Results

Unstructured text is the only MIRAI output.

[Herman Venter]

MIRAI is a plugin for rustc and the latter takes care of emitting the diagnostics. Try RUSTFLAGS="--error-format=json".