Merge branch 'specify-working-directory' into 'master'

Add `-C` option to specify the working directory

The concrete goal of adding this switch is to be able to start a runner as:

```
/opt/gitlab-runner/bin/runner -C /home/gitlab-runner
```

This way we can keep the runner's executable code under `/opt/gitlab-runner`, owned by root, and have `/home/gitlab-runner` (or whatever we come up with) as a writable, working area for the gitlab-ci-runner user.

This new switch does not change the default behavior of the runner.

See merge request !14