Add snippets page

- Add new /snippets page
- Add prism for code highlighting
- Add link to snippets page from GOV.UK elements page
Create snippets for each example
Replace examples with snippet includes

Ensure all examples use snippet includes for both the snippets page and
the index page, this way examples only need to be updated in one place.
Add tabular data example
Remove additional classes

- Only use classes required to describe the example
- Move .text (max-width) wrapper to containing div for examples
Change attribute order for examples

Use the following order:

1. class
2. id
3. data-*
4. Everything else

HTML attributes should be listed in an order that reflects the fact
that class names are the primary interface through which CSS and
JavaScript select elements.

https://github.com/necolas/idiomatic-html#attribute-order

This also makes it clear in the examples which classes are be used to
apply the GOV.UK elements styles for each example.
Format example html

Make indentation and spacing consistent
Replace examples with snippets for index
Fix unclosed paragraph tag
Remove unused example styles

- Examples should only use elements styles
- Remove any styles overriding margin or padding
- Remove unused icon styles
- Remove styles overriding form spacing
- Add missing class to inset content example
- Comment all button styles, these exist only for the layout of the
button examples
Remove inline snippets styles
Compile prism stylesheet for snippets
Update links to code and snippets

- Add external link to Sass files and link to snippets page
- Remove links to icons and images
Add explanatory text for Sass and HTML links
Merge pull request #22 from alphagov/add-code-snippets

Add code snippets