Document second argument of usage(message, opts)
Fix a couple of bad interactions between aliases and defaults

- Alias + boolean + default is broken (#7)
- Default values and options with dashes not working (#20)
Merge pull request #23 from chrisn/update-examples

Change optimist to yargs in examples
Merge pull request #21 from nylen/fix-aliases-defaults-dashes

Fix a couple of bad interactions between aliases and defaults
Merge pull request #19 from Gobie/feature/update_readme

Document second argument of usage(message, opts)
1.2.3