Add support for parsing docs for single constants

The parser already technically supports parsing constants, but the attached PR hooks them up to a new post type and retrieves the associated documentation. We can add post type / editing / display support at a later time in the wporg-developer theme, but it would be nice to be actually storing the descriptions somewhere.

PR: #203

It should be noted that parsing DocBlock template support is not yet available in phpDocumentor, which means core WordPress is kind of out of luck with the current state of constants documented in wp-config.php. Templates are the ability to document multiple elements at the same time, e.g.

/**#@+
 * DocBlock Template example: One summary for all.
 *
 * One description for all.
 *
 * @since 2.6.0
 */
define('AUTH_KEY',         '...');
define('SECURE_AUTH_KEY',  '...');
define('LOGGED_IN_KEY',    '...');
define('NONCE_KEY',        '...');
define('AUTH_SALT',        '...');
define('SECURE_AUTH_SALT', '...');
define('LOGGED_IN_SALT',   '...');
define('NONCE_SALT',       '...');

/**#@-*/

Start with PR #203 and the parser's existing constant-handling path. Check how the PR hooks constants into a post type and retrieves their documentation; done means descriptions are stored, while post type editing and display support remain out of scope. Note that phpDocumentor does not yet support DocBlock templates.