x/image/tiff: implement a generic tiff parser

[tjamet]

Hi,

I am currently working on a project handling tiff based content (proprietary raw from Canon and Nikon, as well as Exif).

There is a library handling my needs regarding Exif but this one re-implements the tiff parser in order to access fields.

I am wondering it could not be interesting to expose a tiff parser in the x/image/tiff package that developers could use to handle content in a TIFF container.

Happy to help if you think this is an interesting idea.

[FiloSottile]

If you want to propose a generally-useful API for that, feel free to turn this issue into a Proposal. See https://golang.org/s/proposal-process