Support phases and optional dependencies

[cyrilwanner]

This is a complete rewrite and resolves #2.
It should get released as v2.0.0 because there are breaking changes and it requires next.js v5.1.x as it depends on features introduced in this version.

• Supports phases
  • Plugins can only be added in specific phases or excluded from phases
  • Configuration can get overwritten for specific phases
• Adds an optional helper to only load a plugin in a specific phase
• Plugins can specify by their own in which phases they should get loaded

[snax4a]

@cyrilwanner Is it possible to use optional with env variable check?
For example include bundle analyzer only if process.env.ANALYZE === 'true'