Don't greedily boldface all strings ending with colons, just the defined labels

If you have any lines later in your Readme that happen to contain colons, it gets pretty ugly.

Example:

    1. Set up the WordPress testing library as described in [Handbook: Automated Testing (http://make.wordpress.org/core/handbook/automated-testing/).

Becomes:

    **1. Set up the WordPress testing library as described in [Handbook:** Automated Testing](http://make.wordpress.org/core/handbook/automated-testing/).

Instead of greedily matching any line with a colon, this patch only matches the labels defined in the [Readme standard](http://wordpress.org/plugins/about/readme.txt).