Schema could include pre/post processors

I'm curious if there is a reason to not include pre/post processors in the schema?

Could add a few more lines after https://github.com/rconradharris/envparse/blob/master/envparse.py#L85:

```
if preprocessor is None:
    preprocessor = params.get('preprocessor', None)
if postprocessor is None:
    postprocessor = params.get('postprocessor', None)
```
