Add imsave function

I found myself reaching for an `imsave` function to compliment `imread`. Presumably this would have similar semantics, and would effectively map over the `skimage.io.imsave` function, or something else in pims.

I don't have a concrete need though, this just came up when writing up an example.

No files or tests are named. Start by locating the existing imread implementation in dask-image and compare its behavior with skimage.io.imsave; then determine the intended API and semantics for a corresponding imsave function, including what completion criteria and tests would be needed.

Written by the indexing model from the issue text.