Provide Command#banner DSL Around Script Description

This more closely matches the look and feel of the other configuration methods, like `option` and `parameter`. The behavior remains identical, and will not interfere with existing usage of `self.description` in current scripts.

See this from examples/speak for usage:

```ruby
class SpeakCommand < Clamp::Command

  banner %{
    Say something.
  }

  option "--loud", :flag, "say it loud"
  option ["-n", "--iterations"], "N", "say it N times", :default => 1 do |s|
    Integer(s)
  end

  parameter "WORDS ...", "the thing to say", :attribute_name => :words
  # and so on
```