Add syntax for example code to docs

Now, code defined with the "example" Markdown attribute will be rendered
as an inline REPL-style example. For instance:

````idris example
with List (reverse ["a", "b", "c"])
````

renders to

        > reverse ["a", "b", "c"]
        ["c", "b", "a"]
Merge pull request #1613 from david-christiansen/doc-examples

Add syntax for example code to docs