Fix 'hidden' equation flag being ignored

[Nigecat]

Fixes the |hidden equation flag being completely ignored.
Using this on an equation will completely hide it (but it can be used if you need to define a function then reference it later without graphing it).
Using it on a point will hide the point (but will not hide an attached label).

```desmos-graph
(4,2)|purple|hidden|LABEL:test
f(x)=x^2|hidden
f'(x)
```

Fixes #91