Add nicelevel/priority configuration option to Runners

There seems to be no way to configure the nicelevel (Unix)/process priority (Windows) when executing a shell in the GitLab runner.

In know the workaround is quite simple by prepending any command with "nice" in the pipeline, but this solution won't work cross-platform. Also, sometimes one runner needs to be configured differently than the next one (dedicated build server vs. shared server).

My suggestion is to add an option to config.toml named priority in the [[runners]] section. For me, three to five levels would suffice: normal, lower, lowest.