Allow the default time-limit for individual commands to be overridden

I've added an option to the config to override the default execution timeout, as I have builds which were failing with

ERROR: CI Timeout. Execution took longer then 3600 seconds.

This may resolve issue #1193 (closed)

I feel there may be a better way to implement this, such as replacing the hard-coded value on https://gitlab.com/c.d.bishop89/gitlab-ci-multi-runner/blob/master/commands/exec.go#L203 with the value from a config, but I couldnt see a way to access the configOptions.config in that context.

I felt the config made more sense in the common/config file rather than being a runner specific config. (but maybe thats wrong?)

Happy to re-work this if there is a better way.