Issues with `mixer.SKIP`

I noticed in the tests, and the source code, that there is a `mixer.SKIP` property that can be used to skip the generation of certain fields: https://github.com/klen/mixer/blob/6af15d5e65d8a2ee8185f7777bf095d611ba13ae/tests/test_main.py#L287-L291.

My first question is, why is this not documented? It seems to be tested so it must work for normal use.

My second question involves using `SKIP` with the SQLAlchemy backend. I've noticed if you try to use these together, you'll get an error:

```python
# I think the model you use here can be anything that has a relationship field
x = mix.blend(models.ReportMeta, report_meta_key='key_1', report_meta_value='1', report=mix.SKIP)
```
And you get:
```
Traceback (most recent call last):
File "/home/michael/Programming/MegaQC/venv/lib/python3.6/site-packages/mixer/main.py", line 568, in blend
return type_mixer.blend(**values)
File "/home/michael/Programming/MegaQC/venv/lib/python3.6/site-packages/mixer/main.py", line 140, in blend
target = self.postprocess(target, postprocess_values)
File "/home/michael/Programming/MegaQC/venv/lib/python3.6/site-packages/mixer/backend/sqlalchemy.py", line 76, in postprocess
setattr(target, name, value)
File "/home/michael/Programming/MegaQC/venv/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 262, in __set__
instance_state(instance), instance_dict(instance), value, None
File "/home/michael/Programming/MegaQC/venv/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 975, in set
value = self.fire_replace_event(state, dict_, value, old, initiator)
File "/home/michael/Programming/MegaQC/venv/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 998, in fire_replace_event
state, value, previous, initiator or self._replace_token
File "/home/michael/Programming/MegaQC/venv/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 1407, in emit_backref_from_scalar_set_event
instance_state(child),
AttributeError: Mixer (): 'object' object has no attribute '_sa_instance_state'
```

I assume this is a bug?

Start with tests/test_main.py around lines 287-291 to understand the existing mixer.SKIP behavior, then inspect mixer/backend/sqlalchemy.py at the postprocess location shown in the traceback. Reproduce the ReportMeta example from the issue and determine the expected SQLAlchemy behavior; done means SKIP no longer raises the shown error and its supported use is documented.

Written by the indexing model from the issue text.