Add filter to add possible template paths.

Currently there is no way to define a template path in a theme and theme developers have to add wp-property specific templates to the theme's root directory.

Adding a subdirectory in a theme keeps wp-property's files more organized.

This filter allows a theme to define another template path to look for templates.

Here is how I using this filter in a theme I am developing:
```php
function filter_wp_property_possible_template_paths( $paths ) {
	array_unshift( $paths, get_template_directory() . '/wp-property' );
	
	return $paths;
}
add_action( 'wpp_possible_template_paths', 'filter_wp_property_possible_template_paths' );
```