Contributing new join strategies

[mhkeller]

It would be great if joiner supported different types of joins. There's still some work to do on breaking things up so there is a common standard. I'll track the progress here:

• [ ] Break out the existing left join strategy into separate file
• [ ] Document the inputs and outputs for a join module
• [ ] Document any conventions such as how to handle null values
• [ ] Restructure tests into separate files so it's easier to add them on a per-strategy basis