Allow generation of story .json files into a subdirectory.

Problem/Motivation

Currently *.story.json files are generated into the same SDC directory as the *.story.twig files from which they are derived.

It is common for generated files to be written into a subdirectory where they can easily be ignored by GIT or hidden from IDEs. (For example: `src/*.js` files build or minify to `dist/*.js` etc.)

Proposed resolution

Provide an optional argument to `storybook:generate-all-stories` that is the subdirectory to which story.json files should be written.

Example: `drush storybook:generate-all-stories dist --omit-server-url`

• /components
• /button
• button.story.twig (source file)
• /dist
• button.story.json (generated file)