Custom field not getting parsed using alias
When will be next release?
 I'll create one soon.

Hi, I have a custom field definition like:

```
class CustomField(peewee.Field):
    def coerce(self, value):
        return "parsed"

class A(peewee.Model):
    f = CustomField()
```

When I parse custom value to Models directly, it works fine.

```
>>> A.select().where(A.f == 20)
<class '__main__.A'> SELECT "t1"."id", "t1"."f" FROM "a" AS t1 WHERE ("t1"."f" = ?) ['parsed']
```

But, when I use alias, it won't parse the value for me.

```
>>> a = A.alias()
>>> A.select().join(a, on=(A.f == a.f)).where(a.f == 20)
<class '__main__.A'> SELECT "t1"."id", "t1"."f" FROM "a" AS t1 INNER JOIN "a" AS t2 ON ("t1"."f" = "t2"."f") WHERE ("t2"."f" = ?) [20]
```

What I am expecting is:

```
<class '__main__.A'> SELECT "t1"."id", "t1"."f" FROM "a" AS t1 INNER JOIN "a" AS t2 ON ("t1"."f" = "t2"."f") WHERE ("t2"."f" = ?) ['parsed']
```
