OME-TIFF extension

[OME-TIFF](https://docs.openmicroscopy.org/ome-model/5.6.3/ome-tiff/) is a microscopy extension for TIFF. I don't know microscopy data at all, but I'd love to support others to build support around async-tiff. Broader use of async-tiff could ensure the stability of the library.

I'm assuming that, similar to GeoTIFF, OME-TIFF defines a set of extension tags, and implementing support for OME-TIFF means implementing a parser for the tag data.

The groundwork for this would probably be in #100, so that Geo and OME TIFF could be defined as extension points rather than being tied to the core implementation of the library.

If anyone is interested in working on this, please comment and I'll try to finish #100

Read issue #100 first to understand the proposed extension-point groundwork, then review the OME-TIFF specification linked in this issue. The work would be complete when OME-TIFF tag data can be parsed through that extension mechanism, but the issue does not identify specific files or tests.

Written by the indexing model from the issue text.