refactor(style): enhance sass customizability

[rbarilani]

related somehow to #25

• use "brilliant" orange (same color of the logo) as primary color by default
• publish also .scss source files on npm to allow importing sass version and let the user"compile it", mostly to override variables or reuse them
• move entry point for sass in the the top folder so is easier to include

why?

Sometimes you want to tweak the style of the theme and a nice way to do it is using sass + hexo.
This PR makes it possible. When this PR is eventually merge and published the user can:

install the sass render

$ npm install hexo-renderer-sass --save

tweak and compile the style, with:

source/style/doc.scss

$doc-color-primary: red;

@import '../../node_modules/hexo-theme-doc/doc';

this will override the orginal stylesheet provided by the theme (style/doc.css) with user tweaked and compiled version where the primary color (eg. for navigation links) is "red".

[rbarilani]

👍

[bhaskarmelkani]

👍