Allow users to specify a custom path to the wp-config file.

Similar to @markjaquith skeleton project at http://github.com/markjaquith/WordPress-Skeleton I use a setup with WordPress in a subfolder. One difference in my setup though is, that I create a symlink to the actual WordPress installation. 

Now, the problem with the current version of locate_wp_config() is, that it tries to find the wp-config.php in the root of the WordPress installation or one folder up in the hierarchy. As a consequence it will only find a custom written wp-config.php which determines the path of the projects actual wp-config.php file, resulting in wp-cli complaining about a "Strange wp-config.php file", where the "wp-settings.php is not loaded directly".

A quick solution to this problem would be to allow users to specify a custom path to the WordPress configuration file as implemented by this pull request.

Example configuration for a project called 'projectname':
```yaml
path: 'wp'
wp-config-path: '/Users/username/Sites/projectname'
```