Add SELECT columns to a preexisting query/ModelSelect 
Looking through the source code, I see I can do this work around by accessing a private attribute:

```
query = Tweet.select(Tweet.id)
query._returning.append(Tweet.content)
```

But I would rather not rely on the private attribute.  Nevermind, I found it:

```
query = Tweet.select(Tweet.id)
query = query.select_extend(Tweet.content)
```
If I already have a preexisting Peewee query (ModelSelect), is it possible to add a column to the SELECT statement of the query?

E.g:

```
query = Tweet.select(Tweet.id)
# query.add_select_column(Tweet.content)  # how to do this ?
```

I think it might be useful to have the `select` function of the ModelSelect return a query with a modified SELECT statement; the same thing that `where` and `join` do.

Today, the `select` function returns a new query from the base model, so it is not too useful:

```
>>> query = Tweet.select(Tweet.id)
>>> print(query.select(Tweet.contents).sql())
('SELECT "t1"."contents" FROM "tweet" AS "t1"', [])
```