Migrate to pug

[Surgo]

• [x] Migrate to Pug #39
• [x] Do npm-check-update #49 works fine
• [x] Run with ES6 (harmony flag) metalsmith/metalsmith#151
• [x] Testing node versions are same as Metalsmith
• [x] Rename the repository to metalsmith-pug
• [x] Migrate services (travis, codeclimate, flattr, etc.)
• [x] npm deprecate with a message pointing at metalsmith-pug.

[ahmadnassri]

excellent work! thank you!

[ahmadnassri]

gotta give kudos to @mikemaccana for showing up randomly in a linked SO answer! :)

[ahmadnassri]

@Surgo left some minor notes for changes, let me know if you're able to take a stab at them, otherwise looks good, great job and thanks again!

[ahmadnassri]

@Surgo all done! thanks for your help :clap: