Adding chunk argument (instead of nframes)

_From @jakirkham on June 7, 2017 19:54_

Instead of specifying the number of frames, `nframes` for `imread` it might make more sense to have a `chunks` argument. We could handle this in a couple of ways.

1. Require the non-frame dimensions to be one chunk. (so either `None` or the full length)
2. Allow chunking along other dimensions, but only apply it with `rechunk` afterwards.
3. Chop the data in `_read_frame` somehow so as to get chunks.

xref: https://github.com/dask-image/dask-imread/pull/2#discussion_r120639877

_Copied from original issue: dask-image/dask-imread#6_