Add pair of operations to control current URL context

[neocotic]

Users should be able to use URL-specific variables and operations but I can only see this as achievable by adding a pair of new operations that allows the user to change the URL in the current context. For example:

{#switch}http://example.com{/switch}
{host}
{revert}
{host}

... would print:

example.com
github.com

However, we'll also need to consider nested switches and reverts so this is likely more complicated than it would seem, probably requiring some sort of stack.