Allow Drush and SSH to accept remote commands as arguments.

Via a new [ArgvHelper](https://github.com/pjcdawkins/platformsh-cli/blob/drush-ssh-arguments/src/Helper/ArgvHelper.php) class, this extracts out the Symfony/Platform CLI-specific options and arguments from the command line, and gives you a command that you can pass through to `ssh` or `drush`, suitably escaped.

So you can run any Drush or SSH commands, while still specifying platform options such as `--environment`.

Example working commands
```bash
platform ssh ls -al public
platform ssh --environment master tail -f /var/log/access.log
platform ssh mysql -Dmain --host=database.internal --execute='show tables'
platform drush vget site_name
platform drush --environment master vget site_name
```

 - [x] remove --ssh option for drush? or switch to SSH by default and remove the alias method? - no, it's useful as an option

## N.B.
In order for Platform options such as `--environment` to work at the end of the command, you need to pass the entire command in quotes.

So this works:
```bash
platform ssh --environment sprint-2 ls -al
```
while this doesn't (or it uses the wrong environment):
```bash
platform ssh ls -al --environment sprint-2
```
but this does:
```bash
platform ssh 'ls -al' --environment sprint-2
```