CSF: Promote args/argTypes to first-class metadata

[shilman]

Issue: N/A

What I did

Promote arg/argTypes to peers of parameters in CSF for both kinds/components and stories.

• Update CSF loading logic (and renamed)
• Update unit tests and stories
• Update documentation

Before:

Example.story = {
  parameters: {
    args: { ... },
    argTypes: { ... },
  }
}

After:

Example.story = {
  args: { ... },
  argTypes: { ... },
}

Note that under the hood they are still stored in parameters. This simplifies the implementation, preserves compatibility with parameterEnhancers, and preserves back-compat with the storiesOf format.

How to test

See attached tests and stories

[tmeasday]

LGTM ++