Add Command Line Help

[vjkoskela]

Aside from documenting the command line options from #1 in README.md the metrics-generator should be extended to offer command line help. First, via an explicit argument (e.g. --help). Second, although most options should have reasonable defaults, I believe we should at least require the user to select an explicit mode of operation (e.g. continuous vs non-continuous) and if not default to showing the help.