Several changes

- Updates README.md with instructions on how to contribute.
- Adds a `.editorconfig` file.
- Tidies up the example HTML and JS files.
- Minifies template HTML when generating $templateCache code.
- Namespace $templateCache module names (`angularChartsTemplate_left`, `angularChartsTemplate_right`)
- Refactor CSS out of the template files, into a SCSS file, which is minified and a `<style>` element is created by the `dist` JS.
- Add a config option, `htmlEnabled`, to legends to allow HTML in them.
- Misc tidy up / code cosmetics.