Switch `exec()` to use bash by default

This should be possible via the `shell` option (see [the docs](https://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback)). I've written out some preliminary code for Linux, and it seems to work as expected. I'll fix Windows compatibility and send out a PR.

Edit: `bash` should be the new default on Linux, `cmd.exe` should remain the default on Windows.
