This PR adds a new static method `Post::clone()` which takes a `Post` instance as an argument and creates a new shallow copy of that Post. The copied Post is then returned from this method.
Reviewed-on: https://codeberg.org/vlw/wp/pulls/18
With this PR we can (among actually creating taxonomies and terms) assign and remove terms from a post with the following
```php
$post = new Post(123);
$term = new Term(321);
```
Add a term to a post:
```php
$post->add_term($term);
```
Remove a term from a post:
```php
$post->remove_term($term);
```
Reviewed-on: https://codeberg.org/vlw/wp/pulls/6
