feat(WPLoader) read content locations from config

[lucatume]

Fixes #745

When possible, read content, plugins and mu-plugins directories location from the WPLoader module configuration.
Respectively, in order:

• content dir from configuration WP_CONTENT_DIR parameter, then from the installation
• plugins dir from configuration WP_PLUGIN_DIR parameter, then from the WP_CONTENT_DIR parameter, then from the installation.
• mu-plugins dir from configuration WPMU_PLUGIN_DIR parameter, then from the WP_CONTENT_DIR parameter, then from the installation.

The following methods, used internally by the WPLoader module are affected:

• getPluginsFolder
• getMuPluginsFolder
• getContentFolder

The methods will now take configuration parameters into account allowing a test override of the default location.

The PR will not, and cannot, deal with WordPress installations defining the WP_CONTENT_DIR, WP_PLUGIN_DIR and WPMU_PLUGIN_DIR constants in their wp-config.php file; in that case the module will throw an exception.