Fix incorrect "unused custom variables" error

[unitof]

Closes #523

Proposing this as a quick fix as I don't see why hbs files in partials/ are ignored for this test.

See #523 for specific examples.

However open to other approaches if there's a reason partials/**/*.hbs is ignored!

[unitof]

Nudging for feedback on this.

[unitof]

@sagzy could you take a look at this? Would really help us get rid of the red error box on our admin, for using a custom variable in our CTA partial (an example from Ghost's own documentation).

We're on Ghost(Pro) so I can't use a gscan fork.