Add regex to match the story's kind

[olistic]

What do you guys think about having these two config options: storyKindRegex and storyNameRegex instead of the current storyRegex?

storyNameRegex would behave just as the existing storyRegex, while storyKindRegex would match against the story's kind, giving users more flexibility.

For example, it could be used to solve #84 doing something like this:

// SomeComponent.stories.js

// ...
storiesOf('SomeComponent', module)
    .add('foo', () => {
        // ...
    });

// SomeComponent.test.js

import initStoryshots from 'storyshots';

initStoryshots({
  storyKindRegex: /^SomeComponent$/,
});

Which will cause storyshots (or jest actually) to output a __snapshots__ directory colocated with the component and with just its storyshots.

If you think it is a good idea, I'm happy to submit a PR!

[olistic]

I guess I'm submitting the PR anyway, it's simple enough and I need it for my projects.

[alex35mil]

Why not just writing snapshots next to the stories file? W/o dummy regexp for each component.