Implement CSS and JS Panel Plugins

[bmuenzenmeyer]

pattern-lab/edition-node-gulp#8
pattern-lab/edition-node-gulp#7
#330

[bmuenzenmeyer]

Pattern Lab 2.6.0-alpha should now allow motivated individuals to create plugins. The behavior you describe could be done with such a plugin.

Read more: https://github.com/pattern-lab/patternlab-node/wiki/Creating-Plugins

This issue has been actually implemented as the first plugin: https://github.com/pattern-lab/plugin-node-tab