Run Python/Ruby scripts for hooks instead of Bash

All the example plugins I see are 100% bash scripts, but it would be really useful to break into another language like Python or Ruby to be more productive.

I tried making a plugin with a simple Python script but see things like this, where it's running each line as bash rather than respecting the hook file's shebang:

```
/var/lib/buildkite-agent/builds/default-ubuntu-1804-vagrantup-com-1/my-org/my-repo/.buildkite/plugins/my-plugin/hooks/pre-command: line 3: import: command not found
--
```

I set `BUILDKITE_SHELL` to `/bin/bash -eo pipefail -c`, which seems pretty close to the default.

@lox any thoughts? :smile:

Start by tracing how the plugin hook at .buildkite/plugins/my-plugin/hooks/pre-command is invoked and how BUILDKITE_SHELL affects that invocation. Reproduce the Python shebang failure, then verify that hooks written in Python or Ruby are executed by their declared interpreter rather than Bash.

Written by the indexing model from the issue text.