Add --only-pending CLI option which filters to only the pending examples

[jvon1904]

Add --only-pending CLI option filter

The --only-failures CLI option is very helpful. It looks in the examples.txt file for tests that failed and only runs those.

Having the option to run only those examples that were pending would be helpful as well.

[pirj]

Firstly, this might be a deceiving option. Some examples would only appear as pending after actually running them, eg those where pending is set inside the example.

Also, this can already be achieved by using the tag filter, as pending examples are assigned the pending: true metadata..

[JonRowe]

@pirj the example status we record is a result of executing the example so includes those where pending is set inside the example, thus this is not currently achievable, @jvon1904 I'd review this if you'd like to tackle it

[jvon1904]

Sounds good, I will work on it. Thank you @pirj and @JonRowe!