Support nested and custom fixture paths

[ovidiuch]

This PR adds support for __fixtures__ folders inside a nested component structure (E.g. components/Button/Button.jsx – @BrunoDF was having trouble with in #182), but also for custom fixture paths for cases where fixture files are kept separately from component files.

In addition:

• It improves on the loader/playground separation created in Isolate loader inside iframe #232. I returned to a single webpack config setup because it made more sense to reuse config between pages since they both load user fixtures which can import any module from the user's codebase. This also fixes the issue where the playground wasn't rebuilding when components/fixtures were added/edited.
• React dev tools gets wired to loader iframe as well, which makes it as easy as when everything was in the same frame to debug loaded component
• Playground looks 1% sexier

@flavius-tirnacop-hs @RadValentin please review. I'll refrain myself from making any other big PR soon! This one has almost as many removed as added lines anyway. Thanks!