Add -h and --flow (on|off|keep) to run-*-test-suite (#187)

With `--flow (keep|on)` run-parser-test-suite will output:

    +MAP {}
    +SEQ []

run-emitter-test-suite will then emit flow style collections if requested:

    echo 'foo: [bar, {x: y}]' | ./tests/run-parser-test-suite | ./tests/run-emitter-test-suite
    echo 'foo: [bar, {x: y}]' | ./tests/run-parser-test-suite \
        --flow keep | ./tests/run-emitter-test-suite --flow keep

Also: add that yaml_private.h include again that I had thrown out. Needed
for printing directives.
Wonder if there is a way to create a directive without using the private api.