Allow command modules to define all command properties

[nexdrew]

This just expands support for command modules such that the command string and description can also be defined by the module, like so:

// foo.js
module.exports = {
  command: 'foo <bar> [baz]',
  describe: 'foo all the things',
  builder: function (yargs) { return yargs },
  handler: function (argv) {}
}

// cli.js
var argv = require('yargs')
  .command(require('./foo.js'))
  .argv

A command string must be provided by the command property (and it must be a string).

A description is optional. If not provided, the command will be "hidden". If provided, it can be a string or a boolean, and for consistency with the options API it can be provided by a describe, a description, or a desc property (in that order).

I see this as an incremental step toward allowing command modules to be more self-sufficient and auto-discovered.

Note that I added a bunch of "API" tests to test/command.js to account for what the .command() method should accept as args, but I'm not sure they provide much value. But I figure, why not.

[nexdrew]

CI failures seem to be red herrings.

npm install failed on Travis, but only for Linux Node 0.12.

AppVeyor failed due to npm 1.x with Node 0.10, which will be fixed by #413.

[bcoe]

@nexdrew mind rebasing with:

#413

should help some of the CI weirdness.

[nexdrew]

Rebased