CF style 'bounds' for histogram output?

[jbusecke]

I would find it very useful to preserve the input bins as CF bound coordinates in the output. I can try to put in a PR some time soon if folks think this would be useful.

[dougiesquire]

Thanks @jbusecke, I think this is a great idea! There are ongoing plans to move xhistogram into xarray (cc @TomNicholas). But, given that your proposal doesn't sound like a huge amount of work, I think it's worth implementing in xhistogram in the meantime. A PR would be great, thanks!