Default values on options are printed in the help message

When a user provides a default value for an option, e.g.

```ruby
  on :n, :name=, 'Your name', :default => 'Lee'
```

Show the value in the help message, e.g.

```
"    -n, --name      Your name (default: Lee)"
```
Merge pull request #143 from KushalP/show-default-value-in-help-message

Default values on options are printed in the help message