Add the ability to start and exec into a devcontainer (docker run -it)

I would like to be able start and exec into my devcontainer with a single command (like you can with docker using `docker run -it zsh` for example).

So instead of:
```sh
devcontainer up
```
and
```sh
devcontainer exec --workspace-folder . zsh
```

There would be some alternative flag/command/whatever. It could maybe look something like:

```sh
devcontainer up -it --workspace-folder . zsh
```

As a side note, the `--workspace-folder` flag is a little unwieldy. It would be nice to at least have a shorthand.