- Add 'required' option support:

    option "--foo", "BAR", "Some description", :required => true

  Notes:
    - :required and :default are mutually exclusive as a sanity check.
    - :required conflicts with :flag options since that doesn't make
      sense either.
- Add specs to cover #23 - :required options!
- Small improvements to the env specs
Update README to document the ':required' feature
Improve multi-line option value and parameter parsing.
Spacing.
Remove talk of environment variables from the "Declaring options" section.

"Environment variable support" has it's own section, further down.
Make the doco around required options pithier.

(No offense, Jordan ... I just want the README to have a consistent "voice").
Fix test.
Merge remote-tracking branch 'remotes/dpiddy/improve-multi-line-handling'
Simplify the tests for dpiddy's patch.
Prep for release.
Hoist subcommand logic out of #execute, making it easier to override.
Let subcommands parse their own arguments.
Override execution behaviour for commands with subcommands.
Demonstrate that #execute may be inherited by subcommand classes.
Crank version forward.

Full minor version change, since my changes to subcommand handling
might have introduced bugs.
Upgrade all the things!
Switch to using RSpec's 'let' for fixture data.
Explicit tests for multiplicity.
Be more robust when inferring the attribute_name of a parameter.

Don't complain about the inability to infer an attribute_name, when
one has been explicitly provided.