[Feature proposal] Variable multi key joining

Hey,

I created a function for joining two frames with different rowkeys. For each frame an additional function is given which maps the rowkey of the frame to a rowkey shared between both frames. This way any two frames can be joined.

The implementation (called Frame.align) and an easy example for this are available in our [DeedleExtensions](https://github.com/CSBiology/DeedleExtensions) repository.

Before creating a pull request, I wanted to first reach out and see if you're interested in this function in particular or any other function included in the DeedleExtensions repository.