feat: add --check_format=json|pretty

Adds a new CLI option --check_format which accepts the values: 'json'
(current behaviour), and a new 'pretty' value which prints a colorized
human readable report to stdout, similar to common compilers and
linters.

Results are printed with color unless NO_COLOR is defined in the users
environment, as per https://no-color.org.

If --check_out_path is provided, then the results are always saved to
file regardless of the --check_format value.