Enhancement: New [yoast_breadcrumb] shortcode

Currently to display the breadcrumb, the user's theme must be edited to include a function call to yoast_breadcrumb() (as per http://yoast.com/wordpress/breadcrumbs/).

This enhancements adds a new `[yoast_breadcrumb]` shortcode, which can be used for displaying the breadcrumb without needing to add a function call to your theme.

This is particularly useful in themes such as WooThemes Canvas, where the hook manager allows you to add a shortcode to the header without having to edit the theme.

There are two shortcode arguments, both of which are optional:

* `before:` The prefix for the breadcrumb, usually something like `You're here`. Can also be HTML such as `<p>`.
* `after`:  The suffix for the breadcrumb. Can also be HTML such as `</p>`.

Examples:

* `[yoast_breadcrumb]`
* `[yoast_breadcrumb before='<p class="breadcrumb">You are here: ' after='</p>']`

Note: If the breadcrumb feature is disabled, then this shortcode outputs nothing.