Support for MySQL STRAIGHT_JOIN
Just do this:

``` python
from peewee import *
JOIN['STRAIGHT'] = 'STRAIGHT'

my_query = SomeObj.select().join(SomeOther, JOIN.STRAIGHT)
```
 This doesn't work anymore in version 3, due to:


```
class Join(BaseTable):
...
    def __sql__(self, ctx):
        (ctx
         .sql(self.lhs)
         .literal(' %s JOIN ' % self.join_type)
         .sql(self.rhs))
        if self._on is not None:
            ctx.literal(' ON ').sql(self._on)
        return ctx
``` Are you sure about that? Seems to work for me:

```python

In [1]: from peewee import *

In [2]: JOIN['STRAIGHT'] = 'STRAIGHT'

In [3]: class Foo(Model):
   ...:     pass
   ...: 

In [4]: class Bar(Model):
   ...:     foo = ForeignKeyField(Foo)
   ...:     

In [5]: query = Bar.select().join(Foo, JOIN.STRAIGHT)

In [6]: query.sql()
Out[6]: 
('SELECT "t1"."id", "t1"."foo_id" FROM "bar" AS "t1" STRAIGHT JOIN "foo" AS "t2" ON ("t1"."foo_id" = "t2"."id")',
 [])

``` The problem is that the STRAIGHT_JOIN is written with an underscore, it's not space separated. Oh well, those MySQL quirks... Ahh, thanks for the explanation. Let me look into it. Yeah, the code doesn't seem to support that very well...going to leave as wontfix for now.
Would it be possible to somehow perform STRAIGHT_JOIN on MySQL tables due to its poor query optimizer except using raw SQL query?

The syntax according to [http://dev.mysql.com/doc/refman/5.7/en/join.html](http://dev.mysql.com/doc/refman/5.7/en/join.html) is:

> join_table:
>     table_reference [INNER | CROSS] JOIN table_factor [join_condition]
>   | table_reference STRAIGHT_JOIN table_factor
>   | table_reference STRAIGHT_JOIN table_factor ON conditional_expr
>   | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
>   | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor
