Add dask._merge to toplevel
There are lots of codes like: `toolz.merge(dask, [x.dask for x in seq])`.

How about adding `_merge` function which can handle `dict`, `Base` and nested list for internal use? It accepts `_merge(dask, seq)`.

```
def _merge(*dasks, **kwargs):
    """ Merge a collection of dask instances and dicts """

    force = kwargs.pop('force', False)

    result = dict()
    for dask in dasks:

        if isinstance(dask, Base):
            dask = dask.dask
        elif isinstance(dask, list):
            # for nested list
            dask = merge(*dask)

        result.update(dask)
    return result
```
