add test option to check if variables in template tags are defined

[Atanazyy]

closes #504

[MasloMaslane]

Is there a reason why this shouldn't be always checked?

[Atanazyy]

Currently some test fail with this flag, because for example in <a href="{{ item.url }}" id="{{ item.name }}" {{ item.attrs }} class="list-group-item list-group-item-action {% active_url item.url %}"> item.name might not be defined and everything works fine. I want to fix things like that and then set this as default check

[Atanazyy]

For now quite a lot of tests fail