Improvements for Grunt Shopify settings.

This PR does a couple of things:

- Updates `grunt-shopify-theme-settings` to `v0.4.5`, allowing the use of custom templates;
- Add a couple of lines to the README about the use of the `grunt-shopify-settings` plugin to generate `settings.html`;
- Add a really simple custom template for rendering the lists of "display location" checkboxes for the sidebar and recommended collection settings.

Hopefully this example and the documentation I've added to to the plugin README on [Customising templates](https://github.com/discolabs/grunt-shopify-theme-settings#customising-templates) should be enough to help you guys out if you want to implement some of your settings sections as custom templates. If not, ping me and I'll try to help :).