Add "dev" mode where main process has breakpoint at first line

Similar to how I can provide `--inspect-brk` with regular Node to wait for my debugger before continuing, I'd love to be able to do the same thing with `electron-webpack`. Right now, my best solution is to re-run `electron-webpack dev` and mash "reconnect" on my debugger in the hopes that it'll connect

Start by tracing the `electron-webpack dev` command and how it launches the Electron main process, then compare that behavior with Node's `--inspect-brk` option. Done means a dev mode can pause the main process at its first line and allow a debugger to connect before execution continues.

Written by the indexing model from the issue text.