API: Consider porting extension accessor from xarray
`xarray` provides a decorator to register a custom accessor on a `Dataset` - see docs [here](http://xarray.pydata.org/en/stable/internals.html#extending-xarray).

It's a bit of a blunt tool, in that it applies to all instances, but outside of that, I found it to be a nice alternative to subclassing or a composition class.

I haven't looked at the implementation, so not sure how hard it would be retrofit onto `DataFrame`

@shoyer - any thoughts or feedback from people using it?