Add help flag to CLI and error handling for unknown flags and arguments

[taphill]

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[taphill]

I mentioned the idea of adding these features to the CLI in discord and wanted to open a PR for it to start getting some initial feedback.

There were a couple of questions I had about flags which seem to be valid, but that I couldn't find any information about (unless I just wasn't looking in the right place):

• debug
• dirty
• eject

Do we want to update the docs and the new --help flag with information about these flags?

Also, within the src/cli/flags.js there seems to be a TODO floating around here. With some guidance, I'd be happy to clean that up since I'm in the file making changes.

[brianleroux]

this lgtm but I'll leave it to @ryanblock to merge up and ship --- I don't remember what debug does but dirty was the original term we used for direct deployment and eject was the term we used prior to --dry-run for emitting cloudformation. I think we want to keep those aliases around but I'm not dying on that hill either.