Handle when scripty is run outside npm run

[searls]

I had previously given no thought to what to do when the $npm_lifecycle_event var isn't provided.

1. When no CLI args are passed, bomb out as unsupported and print usage instructions
2. When scripty is run like scripty test:unit assume that first arg is the lifecycle event name (is this a good idea? Might be risky to encourage people to run scripts outside the contextof all those npm environment tweaks)

[searls]

Decided the best thing to do for the forseeable future is to print a helpful error. Running scripty this way would be an edge case and a vector for support issues that aren't worth worrying ourselves about.