Add linting for .scss files

Try it out by running `npm install` then `node run_grunt.js scsslint`

This adds some (hopefully) sensible rules for writing scss, building
on top of the guidance in [the GOV.UK styleguide for CSS](https://github.com/alphagov/styleguides/blob/master/css.md).

Also fixes the errors it found. In the future we could integrate
linting into our build process.
Update README
Lint for empty rules

Will catch things like:
```
.my-selector {
}
```
Set <main> to display block by default, remove deprecated tags
Add <main> to the list of things that get resetted
Merge pull request #161 from alphagov/lint-sass

Add linting for .scss files