Adding initial framework for a 'doc' command

This is a proposed new command, `doc`. `doc` pulls the PHPDoc for a given function, class, method, or property. I didn't see anything else quite like this, but if I missed something, please let me know.

### Overview

This command uses the [Reflection Library](http://www.php.net/manual/en/book.reflection.php) to pull PHPDoc comments. I'm hoping to get a ball rolling with this PR; as I see it, there's a lot we can add to this, like parsing the docs to display them in a more elegant way.

### Examples

```
$ wp doc get_permalink
Success: 
Documentation for function 'get_permalink()'
============================================
/**
 * Retrieve full permalink for current post or post ID.
 *
 * @since 1.0.0
 *
 * @param int|WP_Post $id Optional. Post ID or post object, defaults to the current post.
 * @param bool $leavename Optional. Whether to keep post name or page name, defaults to false.
 * @return string|bool The permalink URL or false if post does not exist.
 */
```

```
$ wp doc WP_Query parse_query
Success: 
Documentation for class method 'WP_Query::parse_query()'
========================================================
/**
 * Parse a query string and set query type booleans.
 *
 * @since 1.5.0
 * @access public
 *
 * @param string|array $query Optional query.
 */
```

### Notes

* As far as I know, this should work in all PHP 5 environments. I don't believe any additional extensions need to be installed.
* With the recent push to document actions & filters, it would be amazing if we could figure out a way to pull the docs for those too.