[WIP] Scripts for automating tests against multiple Spark versions

This PR contains a collection of shell scripts for automating tests against multiple Spark versions.

`bin/run-multiple-versions` is basically a wrapper around `bin/run` which uses environment variables to change the Spark version in a configuration file.

`bin/run-bisect` is a wrapper around `bin/run-multiple-versions` that allows you to test against every Nth commit in a range of commits.

I've found both of these scripts to be fairly useful in their current form, but there's probably a few changes that we should make before committing this for public consumption:

- [ ] Add a configuration file sanity check that ensures that `useClusterSpark=false` before running; I wasted an overnight run by forgetting to change this.
- [ ] `run-bisect` might be poorly-named, since it's not really equivalent to `git bisect`; I should rename it to something like `run-commit-range`.
- [ ] `run-bisect` should test that the commit SHAs are from the same branch.
- [ ] Add a documentation note to README and an example of how to find the commit of divergence between branches.