Support for `flat` tuples
You can use `more_itertools` to flatten that.

``` python
In [1]: import more_itertools

In [2]: l = [(1,), (2, ), (3, )]

In [3]: list(more_itertools.flatten(l))
Out[3]: [1, 2, 3]
```
 I can use `itertools` as well (which is part of stdlib contrary to `more_itertools`), but this is not a question _how_ to do that:

``` python
In [4]: list(itertools.chain(*[(1,), (2, ), (3,)]))
Out[4]: [1, 2, 3]
```

This is rather a convenient shortcut.
 I'm going to pass on this feature, as it is fairly easy to implement on your own. If you really want, instead of a list of row tuples, a list of scalar values - then you can write either `map(query, operator.itemgetter(0))` or something similar.
 +1, programmers are lazy

While working with peewee we often query database for a bunch of records on a single field and it is handy in this case to have a flat, chained resultset (functionality similar to [values_list(flat=True)](https://docs.djangoproject.com/en/1.9/ref/models/querysets/#values-list))

So instead of having

``` python
>>> User.select(User.id).tuples()
[(1,), (2, ), (3, )]
```

I imagine this could be expressed in API like:

``` python
>>> User.select(User.id).tuples(flat=True)
[1, 2, 3, 4, ...]
```

Can we consider having this on board, @coleifer? I'd work on corresponding PR if this fits the design.

/cc @stas
