cylc get global config: allow retrieving items

This adds functionality to `cylc get-global-config` to allow retrieval of particular items as is done in `cylc get-config`.

I'm adding this functionality with a view to adding entries related to testing in site/user configs and being able to retrieve their values quickly and easily in the test scripts without the need for grepping output from `--print`.

For example, I'm currently looking at adding testing for particular directives so it would be useful to specify a host for that in the site/user config to run it on. If it isn't there then the test will be bypassed. If it is, then the test will run on the appropriate host.