Make some command line parameters and options optional

[morozov]

1. Make the outputPath parameter of the generate-hashes command optional. If it's not specified, the JSON will be written to STDOUT.
2. Make the -o|--output option of the get-impacted-targets command optional. If it's not specified, the targets will be written to STDOUT.
3. Make the -b|--bazelPath option optional. If it's not specified, the Bazel binary available in PATH will be used.

Why this may be useful:

1. Being able to output to STDOUT makes piped processing of the generated results easier. See Log messages should be written to STDERR, not STDOUT #154 for reference.
2. The bazel binary available in PATH looks like a sensible default. It will simplify/shorten the resulting bazel-diff commands in some/most cases.