Allow for using PEP8 compliant comparisons in boolean Fields
There is no way to override the `is` lookup, so this won't work. Same thing for `not`, `and` and `or`.
 @coleifer Would adding something like `.equals(value)` or `.bool(value)` (to http://docs.peewee-orm.com/en/latest/peewee/querying.html#query-operators) be out of the question? I came across this issue because `autopep8` was breaking my queries to be pep8 compliant. For instance, my original query

```python
profile_image = Image.select().where(
    (Image.organization == org_id) &
    (Image.user == None) &
    (Image.id == image_id)
).get()
```
was having its `== None` replaced with `is None`, becoming

```python
profile_image = Image.select().where(
    (Image.organization == org_id) &
    (Image.user is None) &
    (Image.id == image_id)
).get()
```
which led to a `peewee.DoesNotExist` error. I'm not sure if something like @tuukkamustonen's suggestion was ever implemented (I didn't find it), but <strike> an alternative would be to define a constant like


```python
peewee.NULL = None
```
so comparison to null values can be compared with `==` in a `pep8` compliant way.

For instance,

```python
profile_image = Image.select().where(
    (Image.organization == org_id) &
    (Image.user == peewee.NULL) &
    (Image.id == image_id)
).get()
```

</strike>

**Update:** use @coleifer's solution below. 








 To do null testing, use:

```python
Image.user.is_null(True)
```

Or

```python
Image.user.is_null(False)  # IS NOT NULL
```

Which is important because `IS NOT NULL` has different semantics than `NOT (... IS NULL)`.
Current implementation necessitates this sort of syntax:

``` python
     Model.selec().where(Model.field == True)
```

This raises a `E712 comparison to True should be 'if cond is True:' or 'if cond:'` and similarly with comparisons to `False`. 

Would be nice to be able to do:

``` python
         Model.selec().where(Model.field is True)
```

or even perhaps:

``` python
         Model.selec().where(not Model.deleted)
```
