Improve build times by adding node-sass and dev config setup

[Andy-set-studio]

I've setup your Sass to be compiled with a couple of npm scripts. I've also added a setup so that you can have a _config_dev.yml that will override values in your _config.yml for you.

Getting started

• Once this is merged, pull down and run npm install in your working directory
• Create a new file called _config_dev.yml
• Add the following to that file:

# Current site environment
environment: dev

Running Sass

Your main.scss file now lives in your _sass folder. To run Sass, you have 3 commands available.

• npm run sass will compile your Sass to assets/css once
• npm run sass:watch will do the above and watch for changes
• npm run sass:build will compile and minify your Sass and stick it in _includes. You should run this when you want to push your site live

Jekyll commands

To run your site in dev mode, use this command:

bundle exec jekyll serve --watch --config "_config.yml,_config_dev.yml"

It's a bit of a mouthful, so you might want to create an alias or something.

If you run the standard jekyll command, it'll run off your _config.yml file. The same if GitHub pages runs a build.

This PR resolves #98