move plugin examples to `examples/plugins`
https://github.com/assemble/assemble/issues/457
move default extensions to defaults.yml #259
https://github.com/assemble/assemble/issues/259
https://github.com/assemble/assemble/issues/130
Move examples into root folder
https://github.com/assemble/assemble/pull/459
Moved the defaults.yml to lib/defaults.yml and take out the extension specific code
https://github.com/assemble/assemble/issues/259#issuecomment-36756186
Merge pull request #459 from pavelbinar/v0.5.0

move plugin examples to `examples/plugins`