Make config more consistent

[insin]

As mentioned in #43, I was probably over-influenced by trying to keep the per-config item example code small when writing the initial Configuration docs.

Nest everything under appropriate props as per babel and karma config:

• build config object
• webpack config object

Speculative future full-fat config (including changes required to support #43):

var MyConventionResolver = {
  // ...
}

module.exports = function(deps) {
  type: 'react-component',

  babel: {
    loose: 'all',
    optional: ['runtime'],
    plugins: [require('babel-plugin-react-html-attrs')],
    stage: 0
  },

  webpack: {
    loaders: {
      css: {
        query: {
          modules: true
        }
      },
      install: {
        query: {
          cli: {
            saveExact: true
          }
        },
      },
      extra: [
        // ...
      ]
    },
    plugins: {
      define: {
        __VERSION__: JSON.stringify(require('./package.json').version)
      },
      extra: [
        new deps.webpack.ResolverPlugin([MyConventionResolver])
      ]
    }
  },

  karma: {
    tests: 'test/**/*-test.js',
    frameworks: [
      require('karma-tap')
    ]
  },

  build: {
    umd: true,
    global: 'FooComponent',
    externals: {
      'react': 'React'
    },
    jsNext: true
  }
}

Hopefully the need to manually require plugins in config can also be dealt with as part of #41