Update all docs on the usage of pug

[yaodingyd]

Eagle.js doesn't depend on pug and it is discouraging for some people are under the impression that eagle.js does.

• eagle.js
• eaglejs-demo

[atorscho]

Other than that, people who don't use Pug have some troubles reading the documentation. No need to force it.

[ghost]

I also would like to know, how you use the mixins in there?

[yaodingyd]

@johnyscrazy This is already fully documented, as it's how eagle.js works

[Arteneko]

It would be nice to not have pug as default, or at least a setup killswitch to not have it.

[llorllale]

Eagle.js doesn't depend on pug and it is discouraging for some people are under the impression that eagle.js does.

I was not able to run a basic hello world without installing it as a devDependency

[yaodingyd]

@llorllale can you link me to a repo of your setup?