For some schedulers, setting PIMS image reader's `.class_priority` is ineffective in controlling `dask-image.imread()`

cc: @jmdelahanty

Hi dask-image developers!

Normally an end-user may control which reader `pims.open()` uses to load images by simply increasing the `.class_priority` attribute of their preferred `pims` reader prior to calling `pims.open()`. [See this link.](http://soft-matter.github.io/pims/dev/custom_readers.html#plugging-into-pims-s-open-function)

```python
pims.ImageIOReader.class_priority = 100 # we set this very high in order to force pims.open() to use this reader
rgb_frames = pims.open('/path/to/video/file.mpg') # uses ImageIOReader
```

Since `dask-image.imread()` uses `pims.open()`, it would be great if it could mirror such functionality too.
```python
pims.ImageIOReader.class_priority = 100 # we set this very high in order to force dask's imread() to use this reader [via pims.open()]
rgb_frames = dask_image.imread.imread('/path/to/video/file.mpg') # uses ImageIOReader
```

And indeed this functionality *does* work for `dask-image.imread()` in single-machine schedulers, like "threading" and "sync". But I do not know of a way to make all processes, in a multi-process scheduler, for example, aware of the preferred reader's increased `.class_priority`. Any help here would be greatly appreciated.

Alternatively, it might be an idea to modify `dask-image.imread()` to receive a "reader" keyword argument which indicates the end-user's preferred PIMS reader.

Start at dask_image.imread.imread and trace how it calls pims.open(). Compare the behavior described for threading or sync schedulers with a multi-process scheduler, focusing on how the preferred reader's class_priority reaches worker processes. Done means a preferred PIMS reader can be selected consistently, or the issue's proposed reader argument is clearly defined and tested.

Written by the indexing model from the issue text.